1 - The only things that sync on Android are:

• bookmarks
• credit cards
• browsing history
• logins
• open tabs

I believe all of this data is wiped from Mozilla servers if you forget your Sync password. They provide you a recovery key when you sign up: store it safely to retain access to your data in the rare event that you forget your password.

2 - On Android I'm not sure - you'll need to try it out. On Desktop: yes they do.

3 - On Android I'm not sure. On Desktop: yes they do, but usually only if the extension developer has implemented it.

4 - I highly doubt it

2. Not sure it will work, at least soon, as not all Desktop add-ons work on Android. Also I feel like some add-ons are useful on Android while not on Desktop and vice-versa.
3. I have yet to see this work most of the time on Desktop, so I don't know about Android.

Edit: formatting reset the ordered list index to 1 even though it started at 2

1: they do.