1) They do have tactical slots. They are mostly useless unless you just need a small chokepoint/buffer. With TEC, you can actually give them some logistic slots since they have research that adds them to all colonies, that includes dead rocks.
2) It shouldn't. Trade income is credits/sec and just depends on the number of tradeports. I've never noticed position of trade ports change the display of trade income numbers. But I still build them closer to the edge, so the trade ships jump quicker in case there's a pirate raid or something.
3) Yes and yes. Each trade port generates a set income, and they stack perfectly with no diminishing returns.
4) When you're setting up a game, go into Game Options and toggle them off. This was added in 1.03 if memory serves, so if you don't see the Game Options button update your game 
The latest final build is 1.05, or 1.095 beta.
