I understand the benefits of incorporating parts of other religions, such as the creation of the Christmas holiday. But that is mostly harmless to the actual doctrine, what the Taiping Rebellion did changed some vital elements of Christianity, and at that point it's not even Christianity. Keep in mind, the apostle Paul actually advocates being like who you are trying to reach (to a certain point) but, for example, saying he is the brother of Jesus is full-on heresy, and more importantly ludicrous. Jesus is God according to our belief, therefore he would have no brothers save for his human brothers, who died long ago.
Well, if I understand what you're saying I can't really say for certain if that's even true. In my copy of Genesis God refers to himself as a "we". This may be something done by newer translations and not be present in the old texts mind you, but if it's true it would set a precedent for the trinity. Why he would reveal the trinity later and still yet withhold the other potential parts of him makes little sense to me. Not to mention one must question what exactly the point of them would be. The trinity is clearly defined in its roles (as clearly defined as a god can be, anyway). The Father is the ruler, the Son is the voice and the savior, and the Holy Spirit is the part of God that resides within us all, according to Christian doctrine. What roles are left for any other "parts" to fill?
And I think the second part of this is referring to general revelation. The idea that in parts of the world where Christianity could not reach, the message of God is spread to people in similar forms, and worship of that is accounted as worship of him. It's a real recognized thing, and there's much debate over its credibility, and I'm not sure if it applies here. I don't actually know a whole lot about Taoism to be honest, so I can't say for certain how compatible the religion as a whole is with Christianity.
Interesting view about new branches in Christian believes, valid with in itself. As expected. The fact most history book enforced the idea about his claim of being Jesus's brother is probably not fair for him. He lived in an area where you either die hungry or riots against the government to live, and the only hope is by spreading a believe system to others who don't understand the concept of spiritual discovery, and only want foods or physical promises.
So some physical promises about heaven on earth is probably a better idea. And he had to form a new kind of government - rather than bounded by blood, but by believes in God. So some form of ideology to let the people understand his stand in this system is needed. And since traditionally the emperor is consider the son of the heaven (descendent of deity), but if he did stay true to Christianity he can not claim that, because it's Jesus. And there is no pope of any other authority can give him the recognitions he required in China. But if he is only a mortal, than he is no better than any other prophets. As a ruler, he can't command the 100 million people by just say his has a vision, people always believe that blood tie is more important. And he can't be Jesus's son either, so he chose a compromise. Told people he is the son of God, but not Jesus, and it lead people to think it automatically means the relationship is brothers. But the claim about being children of God is valid after all if it is more spiritually than physical sense. He actually could mean he is a spiritual child of God, as everyone else. As everyone has a holy spirit in them. But the record of his words is not well documented as he alternately failed, (the one actually survived never mention anything about being brother. He even criticized in this work about the validity of Chinese emperors, since they do claim to be the son. And he helped translate old/new bible into Chinese, and adding some praises in the form of poems only, ). Also most verbal recorded are heard from survivors of those 100 million followers, at least 20 millions civilians killed/slaughter by soldiers, highest count up to 70 millions,(look at that death tolls, you can imagine the horror), those who survived probably has to say something bad about 洪秀全, and said he is a lunatic, exchange for be spared after the slaughters from government's army. What's really in he mind, no one can tell for sure after such chaos. (It's a very heavy and sad part of Chinese history, and probably result in bring down the last dynasty of China). He is probably more of a tragic missionary/prophet than a leader of heresy cult.
I do have questions about trinity though, as I recalled, there is no where in the new/old bible actually says anything directly about trinity, just indirect reference like naming Jesus Christ, God, the Holy Spirit in one sentence. Or in the old bible, appearance in the form of three man, but that's even before Jesus Christ. And the number could means many than actually number of 3. But God do present different form to different person. As far as we can tell, if Jesus is one of the old 3, than it certainly possible God using different form in different time. So even if it's true there is trinity, it can still be many incarnations be documented in the history, if they are past down by generations of story telling. Or god in fact are more than 3 forms can perceived. Or even with a simpler explanation, that the holy spirit in us human, manifest as a form perceived differently by different people, so all the deities are just our reflection of that, and Jesus still walk on the earth once. Still compatible to Trinity as well. I guess Trinity is much a fixed concept, Christian today no longer question it anymore, since early churches "set" their rules about trinity, but as long as you are not Catholic, you don't need to comply to the authority of Pope as only representative on Earth, or listen to their words as its valid as Bible itself. And the point of many faces is obvious, do you think Jesus walking in China can spread the word of God? We looked to much different! At least be user friendly to the potential believers / your children, and if you are all powerful, this doesn't hurt or hard or violated any of the teachings. Being good, following the instructions of the God, and probably much easier. (You can imagine a white Jesus says we are all brothers to a group of Chinese people? And the most probable reaction to that?) And if general revelations can not be apply, than only the selective few (special), than it will be a very restricted religions. In the form of direct revelations, it will be saints and prophets are important in such conditions, and such equal to my original theory about multiple possible way for prophets to perceive and in different cultures, it will certainly be very different. And if it was done on the time of old bible, there are hundreds thousands of nations in China at that time, it certainly will cause many many difference interpretation of a single God.
About Taoism, its a rather complicated topic. The deities roles in Taoism evolves over time. At first it maybe very close to Buddhism philosophy, means no deities at all, but we can be the one with the universe, if you follow the way of the nature. The concept of creators/managers/heroes ascension into deities ranks, is the result of combining folk religions. Since there are many nations in ancient China before unification in 3rd century BC, there many many cultures and their god/gods existed. When society melt together, the folks who practice them didn't stop them, and early religions most certainly will be polytheism. But combining with the idea of ascension into equals as the universe, through 'Tao'. It construct a similar believes as Hinduism, That since you need to advance steps by steps (We still have the old sayings 循序漸進, means "step by step in an order slowly"), so its easy to imagine there are steps between as human and the almighty universe. And people/spirits/true self(真身) who are half way there will be super beings as powerful as deities can be. Less powerful when you are recently join the family of deities, and many great heroes in real life will be memorized, and combining with the ancestral worship practices and ranking government system from Confucianism, it make sense to let these historic figure into deities as well. The older you are in history, the more great legends you left behind, the higher the rank, and powerful you are. And those fictional figures may as well joins in, since they are probably some ancient deities who manifest themselves in the mind of humans to spread their words. Hence it becomes a gigantic collections of deities list / steps, and practices you need to follow in order to be chosen by these deities, and hoping one day you can become one.
So in a sense, there is only one God - the universe itself. And those deities are spirits ascension to heavens. But instead of staying there, these spirits will come back to earth, for helping their children' children's children... And, its easy to understand why folks can accept why their prayers don't come true sometimes (maybe most of the time), since 'deities' are not all powerful as true 'deities' should be. And true God never bothers little things about humans, since there are too many steps between human and God, he need to manage and listen to other deities wishes first, and assigned those jobs managing humans to them. You can perceive it like it's a human->saints->angels->God ladder, but instead of pre-chosen roles, and with much more refined roles and steps from man to God, people's soul can move up (or down, its possible for anyone even deities to get 'demoted, even ranks below human, as in Hell), all until they are one as God. And the way of getting promoted is by the doctrines of following the 'Tao'. Mostly good behaviors, karma concept borrowed from Buddhism, and secret way of training. It's quite an addictive believes really. Like playing an RPG game leveling up. (Which in term this concept resembles the castes system in Hinduism, but instead divided people in ranks, Taoism divided deities in ranks, as humans are all equal in ranks)