1a.) Depends on how eager you are to play.  No one can answer that one except you.
1b.) Rebellion has all of the content of Trinity, but it is my understanding that you cannot use it to play each version separately.  So if you want to play a mod made only for Entrenchment like 7 Deadly Sins or play online against others who only have Vanilla Sins, for example, you would have to buy Trinity anyway.
1c.) We were told Spring 2012.  That's as close to accurate as we can get at this time.

2.) The pre-order for Rebellion is through Stardock, not Impulse, or Steam, or anyone else.  Though it will likely be possible to buy on those DD services as well when its much closer to the release date.  Alternately it has been confirmed that a retail box version will be available so you may choose to not pre-order at all, or pre-order and then upgrade that to boxed closer to the release date.  Just putting the options out there.

It was, but they sold it off to GameStop. Made major gaming news a year ago or something like that. It fact I think it is now called the GameStop App Store, "Powered" by Impulse. While I haven't noticed a whole lot of changes, some people do not want to do business with GameStop period, so the fact that the pre order is through Stardock directly is a plus. If you feel strongly about your client of choice it will probably be available on Impulse and Steam later.

If you buy it from Stardock directly, through preorder or otherwise, the game will not be tied to a particular store client (as Impulse is now its own thing). To me this is a plus, and I'd rather not use Impulse or Steam if its an option, though Stardock might release a Stardock store app to help with the updating (but exclusive to their own products this time, unlike Impulse). If you find it more convenient to use Steam because you already use it, that maybe better for you, though as Sins is not a Steamworks game it will of course not have that extra functionality.

All else being equal Stardock gets a more of the price you pay if you go through them, but it really isn't a huge difference where you get it from.

As for whether to preorder or get Trinity, well the most economical thing to do is to always wait. Eventually even rebellion will hit $20 price, but that may take years. It would be easier to make your decision if we knew when the Beta will be launched (that way you can at least start to get something out of your preorder, though do keep in mind Stardock betas are far more buggy/in development compared to say minecraft), which is supposed to be soon considering the game launches Q1 2012, but it wouldn't be the first time Sins has had its publish date moved back... again (better than the alternative which was Elemental). Its really your call, and more than a little bit chance until we know a bit more info.

If you have to chose, you'll wish you'll be very jealous when Rebellion gets released and you don't have it.

Diplomacy though will still have tons of mods and features and if you have Trinity you can be in the beta of Rebellion.

Tough choice.  Best bet--find a friend who will let you play on their account then buy Rebellion yourself 

That's cool--I'd get it now too if I was in your place.  The price is right!

Yes, if you purchased from us, it would either be a standalone install, or through the new Stardock Central that we're working on (which is not required to have running to play, or anything, it's just a downloader/installer/updater)

Technically, Impulse is still required if you have the individual expansions and base game and not Trinity, SO THERE.  

Aaaaah, I see it now.  FINE.  You win...