Having just dealt with compressing a table with both clustered and non-clustered indexes:

i) I don't believe so. I believe you can compress just the indexes, but you probably won't gain much savings there.

ii) No.

iii) I believe it is. I can't say for sure, my job that did the compression of the clustered index doesn't have an "online=on" set, but I also did it during off-hours so I wouldn't affect the users...

Jason