b) He is given say a sixty second chance to rejoin, during which his units simply aren't being controlled, as if he was AFK
I dont know, sometimes it takes me up to 10 minutes for rejoining. The whole procedure is rather slow. Leaving the units uncontrolled for such a long period will be the same than is to lose them altogether. AI is not the best choice but still is better than nothing.
c) Exactly, but you will be forced to find allies before you make your move, thus stopping the invincible Zerg rush that is anticipated - finding allies for an early start is fine.
I can bully my way through. Imagine I say - I'm going to launch. I don't care if I lose, I'll take some of you with me. Join me in obliteration of xxxxxx (take a pick). Well, of course I will lose against 5 players in survivor mode, but there's only one winner and everyone risks being nuked by me.
d) But the idea is that you won't win; there are advantages and disadvantages to early escalation, and if you are skilled enough to take on 5 nations then I don't think that it will be a problem; you deserve victory.
Skill has nothing to do with that. Even luck won't help you. Only divine intervention could

Nobody would win against 5 players in survivor mode.
Everything you'd described can be done in the current version of Defcon (you just need to draw some additional rules which players are agreed to follow to). Moreover - the game will lose its dynamics. You can emulate the game you'd described by using a simple IRC channel (you don't need Defcon to play politics, because it would be just a decoration). I hate to wait for Defcon 4, then 3 when real action begins. The game would become slow and boring. Of course there will be a few fans of that mode, but I don't think it will be very popular.
f) Yep. Realpolitik is just my spin on Diplomacy, giving players rather than a descending time limit the decision of when hostilities start and who loses the least.
If there were no limitaions, hostilities would start as soon as everyone pressed ready

Timers are just a fuse, a delay that gives players time to deploy their units and agree upon the alliances. Once Defcon 1 sounds you cannot resit - you just start using nukes

There's no point in putting any additional limitations on the main feature of the game - the nukes.