Thanks for the definitive answers for questions 1, 4, 5 and 7. Looks like I'll have to buy another external drive, but the system update stuff is good news.
It's weird that no one is reporting about whether Wii homebrew can be installed in Wii mode, but I read that homebrew won't be kept over from a system transfer, so that's question 2 half answered.
For question 6, about cross-game chat, I've read that there's a way to send private messages to specific friends on Miiverse, which could serve as a solution, albeit a very weak one. The lack of this feature, and of party chat entirely, is a bafflingly huge oversight.
Thanks for the impressions on eShop download speeds, and though I'd like hard data on them, it's not a pressing issue for me unless Wii ethernet adapters become hard to find.
The answers on screenshots seem contradictory; can anyone confirm whether you can post screenshots to Miiverse, and give an impression on how robust this feature is (whether you can do it at any time)?
I'm still confused about the last question, but
this post makes one of the big reasons I was asking moot. Still, does this mean that the gamepad must remain on even if I'm playing a game using only a Wii Remote or Pro controller?
Question 3 is by far the most important of the lot for me. I don't understand how an account system exists, especially with another layer of "users" on top of it, but doesn't allow you to use that account on different pieces of hardware. Isn't that the entire purpose of an account? Seriously, I need some clarifcation on this.
The original questions are
here