I am still early but from what I have read and seen so far:
Doesn't appear so. Every chapter you get new people though. Lots of units are thrown at you. One nitpick I have so far (and I have seen other people complain about), all the new units seem to immediately outclass your older units.
Not that I have seen, but opening cinematics show that there could be something related to multiple lives, cycle of conflicts over years.
I am avoiding spoilers, but seems like mid-20s.
Not that I have seen. Single story.
A special class uses knives which can poison. Martial arts class was combined with healer. Martial arts is good against tomes, bows, knives.
There are some interesting changes, but they seem pretty subtle. What I listed in 5 seems like most of what I have noticed. Class upgrades seem to be pretty linear, compared to older games.
Linear (from what I have seen).
Yes. More like the 3ds ones. Trope-y. You have the girl that likes to exercise, the big dude that is really a softy, multiple characters that put themselves down whenever someone praises them. I think this stuff is pretty standard to the franchise in my opinion though. However I doubt it will get much deeper like with 3H.
Yes, exercising. Fishing. Virtual pet stuff. I don't think I have unlocked everything.
Pretty small. You could probably argue that it is only one or two sections of Garreg Mach.