1. Overrides. IE if you have a trade center (15% econ or something) building a bank will give you 24% econ now.
2. Short answer: Yes. Long answer: Yes.
3. Influence starbases max out at 387 Influence I believe so the enemy will laugh at 2-3 starbases with only 2 modules. Not sure about economy bases and trade routes. I have heard that you need the starbase to be near the planet the trade route starts in. I haven't noticed any significant increase in trade income from economy starbases.
4. Upgrade it back to a starport. If a tile is currently upgrading, you do need to cancel the upgrade, then overwrite the tile with a starport.