You cannot compare the capabilities of the Wii U with over half a decade old tech. I'm not saying your implication is wrong or even slightly off the mark, I'm just saying the way you are coming to your conclusion is a poor one.
You're right. There are a ton of unknowns. We don't know how well Nintendo supports 3rd party developers for the WiiU so they very well could be crawling in the dark with awful documentation for the API.
So we have three (very generalized) paths to the conclusion. Either the hardware is crap, or the API/support for the API is crap, or the developers are truly putting no effort into their ports.
It could be any of the 3. The fact that there are multiple ports with issues is a little disconcerting but certainly nothing I can get a surefire conclusion on.
Considering what they got out of the Wii, I'd say Nintendo are exactly this? And I'd also say your underrating JPN developers but that's another thread all together.
Not quite. While they did get quite a bit out of the Wii, there is a HUGE difference between squeezing power out of an aging system vs implementing modern techniques on modern hardware while simultaneously squeezing power from it. Look at what Naughty Dog managed to do on the PS3 from when they first started (Uncharted) to The Last of Us. Clearly there has been a ton of work and TONS of new techniques used. They were still very competent on the PS2 and it still took them years to get to The Last of Us quality.
At this point, you can't really conclude either way about how Nintendo would do. Based on the past, I'd say they will do fine. I don't really think they would be at the same level as high end devs from Epic because the guys at Epic working on UE4 are right on the cutting edge.