RE: Culture doubling/years per turn
One other related aspect is the tourist attraction flag, which lets wonders/sm wonders generate cash after 1000 years. Not a wholly applicable concept to MoM, but same mechanism. As to how long, I've modded my game to a flat 10 years/turn, which seems to work out okay. I never play with the cultural victory condition on, though (it just seems anticlimactic to me). I wouldn't go lower than 5 years, which puts the double point 200 turns after construction , as my games tend to only last 250-350 turns before casting the Spell of Mastery.
RE: Defender stats
They work quite well as they are, imo. Looking at the base game, for example, mech infantry defends at 18, modern armor attacks at 24, so a 4:3 attack:defense ratio is the designers' target. Defense 8 units should be able to handle anything up to 10-12 attack or higher, in favorable terrain (and high attack units slaughter them unfortified in the open, as it should be

.
RE: Nukes
I think the issue with units surviving comes from more and larger HP bonuses than in the base game. I've never figured out exactly how it works, but it SEEMS that nukes deal 2-4 damage to all units in the radius (or a %, like 40-80%, I'm not sure), maybe more in the center square and/or less in the outer ring. That would kill basically every C3C unit in 2 hits, and a good number on hit 1, while especially the trolls could survive 3 in MoM. Still works okay for me, as whenever I go to nuke an opponent, I usually have 50-100 missiles to play with.

I hate nuking, though (usually build that many as a STRONG deterrent to the AI) and never devoted the brain power to nailing down the scheme.
RE: Regicide upgradable kings
One thing I'm twiddling with now, and will be playtesting soon to see if it works, is giving "king" units the "leader" flag instead of "king", and building games as a scenario, where each faction starts out with a level 1 leader (so they can't build more), who can be upgraded if he survives. The AI still would have no clue, but it would alleviate the necessity of using Regicide to get the idea. If it works, I'll look into adding it on to a future release of MoM for you, Drift. I've already worked out decent balance stats and graphic design for the 4 "races", and with a singularly unique unit, balance isn't as big an issue anyway, just adds a little flavor (as long as it's not a 1000 attack God or anything!). I'll let you all know how it works out.
That's about all my backlogged comments from pre-holiday civ-withdrawal. One general question: what's the deal with having to manually select ALL stealth attack targets? Shift-click etc doesn't work on that box. Anyone have a quicker method of selecting the whole list?