I have no idea. I would assume it is like the Marine where you get paid depending on certain things. For example, in the Marines, you would get a bonus for being married or being overseas.
No clue either except that pay is always based on rank. Each rank has a paygrade wich is the same regardless of command, but like Matt said, there are lots of bonus's that you can get.
SOCOM sailors probably get but loads of tax free hazard and immenent fire pay though. Not that it matters until after you get through BUDs anyway... good luck.
As far as missions it would depend on where you're stationed. I was VBSS in the Navy and the Seals were our backup. Then one day they told us we couldn't have Seal backup because all the seals were already busy in Iraqistan, So we had to do a bunch of extra training and got bigger guns so we could be our own backup