Don't see any other reason than sig spam, if he ever gets banned again. Also, do different types of sig spam(for example this was posting multiple times in a row) count as different "reason"?
How about trolling? How about 'bumping spam'? There are 31 rules, and you can get banned for each one. I'm certain that what you asked counts as the same reason.
AFAIK this only applies if you are banned for the same reason.
Usually, yes, but not necessarily.
I believe that the progression of bans usually goes from a 14 day ban to a 60 day ban.
Not necessarily. It is more of a case-by-case basis.