Venezuela, Iran & others: a coalition? (New title)

[AdamG]

Assuming Iran's ballistic missiles could be modified to launch from a sea platform, then what is the purpose of Iranian missile bases in Venezuela?

Gotta do training, storage and logistics somewhere. The missile(s) don't care where they're launched.

Secondly, using a nuke for a high-altitude EMP burst isn't such a great strategy unless it's followed up with something else. What are the Iranian's going to do afterward, besides get their asses handed to them? Under what circumstances would they be willing to light off a nuke above the US instead of on the US?

You're assuming you need a nuke to generate a city-crippling EMP burst. I've read otherwise. See http://science.howstuffworks.com/e-bomb3.htm

Then there are the practical issues of launching a ballistic missile from a pitching ship, transporting missile and nuke from Iran to the US littoral without getting intercepted, ensuring the missile will operate after exposure to a marine environment, etc.

Reference ship-launched ballistic /SCUD missile launches, I've read otherwise.
See http://www.missilethreat.com/scenarios/

Again, it's theoretically possible and we should always keep a close eye on the Iranians, but the risk-reward calculation for attempting something like that isn't a favorable one in my opinion.

You're assuming rational thought amongst our opponents. That can be fatal.
(Or, as Foamy the Squirrel said : "It wouldn't hurt you people to think like a serial killer every now and then...")
See http://www.youtube.com/watch?v=r8eICCKMNEc