Complex Space Battles! Help me out.....

I totally agree with Johan. Just to add to Johan's point though, I think this "scope" problem is very common amongst us game designers. I have recently found that limiting my scope to the most interesting (or what interests me most) aspects of my design and developing THAT has seen better results. Oftentimes, we play a game with numerous "dimension" : a battlefied, a trading dimension, and a civ building dimension, each of these can usually be fleshed out into a game on it's own. For the most part, at least for me, I usually have one really good idea for a mechanic or interaction and start making a game. Then midpoint, I start adding other mechanics or other "dimensions" of play and the game design process becomes unwieldy and uninspired and I eventually drop the whole deal. My current philosophy is to go lean. If I have a great mechanic, I simply stick with that and only add things that interface elegantly with it. If it seems forced, I drop it. Limit the scope to the original and interesting. Everything else is excess fat and should be cut.
Another thing you could do is make a certain dimension only minorly interactive. Combat doesn't have to be interactive. Maybe hte guy with a certain type of army units/power cannot be forced into alliances or can decide the laws enforced, etc but isn't as powerful a trader due to his imposing military power and lack of tradeships. It doesn't have to be actual little ships moving around rolling dice to shoot each other. It could be an elegant part of gameplay with counters, cards, etc. It seems more realistic anyway because if I have a huge fleet of warships, it would be silly for me to be forced into anything and also as silly for the players with alot of money and a small military to want to sell me things openly.

On the other hand, if you want to make a Battle Game (which it seems like you do because of your details for the combat system) then make the other stuff less of a distraction or emphasis.

I have been working on a space boardgame that uses a simple and completely non random combat system. In this system the player has 3 ship types: Capital ships. Bombers and Fighters.

Capital ships were good against fighters but poor against bombers. They had good armor and always got first attack in combat, but moved slowly.

Bombers were good against capital ships as they could launch missiles over distances. They were poor against fighters as fighters could move faster enough to attack the bomber before it coudl fire.

Fighters were fastest moving ships which allowed them to attck bombers before they could fire, however their weak armor made them vunerable to any attacks.

Each ship had a move value, armor/shield value, long range weapon value, short range weapon value and a special combat feature: Capitals had first strike, bombers had ranged attacks and fighters had their fast move speed.

When a combat occured the damage results would occure simultainiously. ie my attack will do 5 points of damage and your attack will do 3 points of damage then both ships would take the damage, then if either was reduced below 0 armor/shields then it would be destroyed and removed from play.

This system has a Scissor/Paper/Rock relationship between the ship types (Fighters beat Bombers beat Capital beat Fighters) giving it a tactical and stratigic aspect. It is also quick - just make your choice and subtract the armor value from each ship.

This system could have some semi-random elements put in: like each player has tactical/strategy cards that they could use to influece the damage done by their ships or give some other special ability to a ship for a period of time, or proximity to bases or other ships could have an influence.