1a) Yes
b) Depends on your playstyle. It allowed you to grind a lot to upgrade your armour before moving on to the next tier, but you never have to do that. Group can be more grindy, if multiple people want the max upgraded armour for some reason, since then you have to widen your gather radius.
c) Depends on how much you rush probably. 50h for "seeing most stuff" is realistic I'd say.

2) Your choice but: Mistlands is mostly "more content" and it's not finished. Stuff might change on the beta branch.