All those lands gained in 78 years with only 14 infamy? I'm not buying it. Royal Marriages can only get you so far, and forming the Empire is hard as it is!
Conquered? I didn't conquer any of it; that is the extent of the HRE prior to full annexation. I entered a war against France early on and used cavalry, aided by the Empire's enormous manpower surplus, to shatter it in most of its subcomponents; another war finished the deed. Meanwhile, Castille annexed Aragon and good deal of southern France, whereas Portugal got took all of Ireland and a chunk of England. I had also helped Denmark, which managed to form Scandinavia; the Teutonic Order had also expanded greatly in the east and formed Prussia; then, by snagging Castille, Portugal, Hungary, Poland and Lithuania in the Empire, I managed to reach the Empire's maximal borders, the chiristian world in a nutshell.
Between freeing member states and defending the sharding's offsprings from their formal owners, I easily got all the Imperial Authority I needed, waiting for 100 before imposing peace through the empire. What followed, after having vassalized all members who were not already my vassals, was a war against the Ottomans, which allowed netted me all of Africa as vassals.
In the actual game, I only unified the Empire around 1700 so that I could keep up to date with tech, being a 3 provinces minor with all of Europe providing tribute, and inherit a fully colonized America; I had all of the continent at my beck and call, so I proceeded to either vassalize or annex Asia, selling annexed provinces to my vassals. The large income, high tech, instantenous stability recovery and immense manpower made the game a breeze. The image I gave you is merely a loaded save where I form the HRE as soon as it had became possible; it is not even remotely profitable to do so thus early, specially so if you consider how many nationalist rebels would appear in the next years.
I was Bohemia, which is a fairly easy start, and spent most of the first century with the 8/8/8 Emperor Ledislav II; I also had quite a few vassals in Northern Italy and Germany, and Austria was disabled early on once I stepped in to protected Milan. Castille was incredibly large, and it did form Spain a few years later, assuming I don't annex it; everyone had a Dishonorable Scum CB on it, but the the no CB Imperial Reform kept everyone from waging war on them.
My inflation by 1578 was at 40%, having burnt through large amounts of ducats in gifts; I was getting 11 diplomats/year between my Ruler and the number of member states, and my King's diplomatic skill of 8 stacked on top of my the Imperial Reform's +1 and Advisors +3 quite neatly.