If the centipede can't swim, the adventure is already over.
Not necessarily right. If we cannot swim and do not float, we should be perfectly fine. We sink to the bottom and run along that bottom to the water edge. There's various creatures and armadillos which can swim, but also often choose to walk along the bottom, and some aquatic creatures like older hippos and almost every type of ocean crab which cannot swim but very much can walk and run along the bottom.
If we cannot swim and do float, we may need to think much harder to find a way back to shore. The most urgent issue is just breathing, and as long as we are floating we can breathe.
You've already put yourselves in an extremely dangerous situation by attacking the Welif and not simply letting it carry you to new sources of food. With luck, you haven't killed yourselves already - get out of there.
Our ride started to submerge as it left shore, so simply riding it to whereever its next destination was does not gaurentee any form of safety for us. Additionally, it itself is a hair triggered ambush predator, and there's no gaurentee that we could have safely left the creature by simply moving away - a danger that we considered when we decided to stay longer and feed, attempting to cause the creature maximal damage in minimal time.
We do need to get back to the surface. We were not necessarily stupid to submerge, with or without spiracles -
http://thedragonflywoman.com/2012/05/21/breathing-underwater/. Insects can and do survive submersion for time periods.
Also, your number one evolutionary priority should be to get a dedicated stomach, mouth and digestive tract. That's why your "metabolic efficiency" is still low - you're digesting inefficiently through the skin on your underside..
We already have that, and personally I think it is a bit redundant. It was one of the first things we evolved, and we've carried it with us, mostly unused, ever since.
Current Organs
Omnivorous Digestive Tract (Sewer Mouse): Simple digestive system with an interestingly large-for-an-omnivore caecum and several organs for increased digestion of food matter..
After that, you need tougher skin. A single scratch to your vulnerable underside will kill you - you won't survive contact with a rat that knows you're there. You cannot hunt while in that vulnerable condition.
Debatable - our ability to digest through our skin is a very powerful tool, and weapon. We've already improved our ability to digest through our skin once, and it's clear it will take a lot of evolution points - but it's possible. Extremely rapid digestion is a VERY effective defense.
We can and have been hunting despite being vulnerable.
Our very early evolution of hair was an attempt to toughen our skin.
Now that we have the welif, more toughening may be possible - but I lean very strongly on the 'preserve our digestive skin' value. I agree totally with stronger digestive skin and think most will agree that's logical - but it may be VERY far down our list of priorities.
We have to consider each need of the moment, and what good options we have. If we have a poor defensive choice and a good something else choice, why pick the bad defense? Later we'll have a better defensive choice, we'll waste the time and evo to rechange, and we still wouldn't have the good something else choice. Better to do what we must then do what we can do well, while working towards having more great choices.
Once you have a mouth, you can make it an appendage that can cause damage. Then we can actually fight.
We've already got a mouth, it's a 'grinder', and it's on an extension of our body most recently refered to as our 'tail'. We don't really have a head, and our grinder happened not to fit into the welif's opening that we used. Our grinder happened to be useless against the welif's superior hide, something the creature is renown for being extremely good at - having a very strong hide. That doesn't mean that our grinder is useless overall, or that we no longer need our digestive skin. We can already fight and have.
As for your hopes for the future, Jetman, I urge us to keep our non-immediate future open. We don't know what is about to occur that will force us to drop our future plans - however I especially disagree with our need for 'reproductive organs'. We have no reason to believe we could control our 'children' if we can in fact produce any. We are of a species that appears capable of 'eating everything organic' - There is no reason to believe we are innately cooperative or even possess a means to communicate with other members of our own species in any way prior to a long period of having successfully consumed the right things and a lot of additional organics to be able to evolve those methods. Everything our young eat we cannot eat - and our kid(s) may well try to eat us.
As to our lack of specialized organs and our need for those.... that's actually an assumption. We handled the 'nasty chemicals' from our Fermroot meal just fine. We actually do not know much about our circulation, how we break down molecules we are digesting or which could harm us - we may already have perfectly good means of doing many of these things.
I hope we do take our sewer mouse's sense of smell one day.