1. It boosts your economy and theirs too. Think of it this way: you're helping yourself and ONE enemy build and research stuff faster by making more money. That's good if you're in a 3-player map because generally you are better at spending your money wisely than the AI's. If on a 10-player map, it's MUCH better to give one player and yourself an edge than keep all 9 enemies on the same level. [Note: if you want to be a backstabber, dump the excess cash into bounty on your trading partner. Aah, the joys of being evil... ]

2. The longest chain is not the greatest travel distance. It's more like the optimal number of hops to traverse your empire from one side to the other. So if you have a chain like this:
*----*---*---*---*

And then you conquer this planet
*----*---*---*---*
.................
..*

That doesn't mean you'll get this as a trade route:
*----*---*---*---*
.\..............
..*

Instead, you might get this.
*----*---*---*---*
..../...........
..*-

3. Correct. It's most effective to place refineries in gravwells where you've got a huge bunch of exit phase lanes.

4. Vasari are best at spamming the mineable resources. Upgrade your research on resource production early and then start putting your stuff on the black market when you start getting too much of it. But don't sell it at the going rate. Right-click the lower-left multiplier buttons once to set your standard price just below market value, then click the buttons next to them to put your metal and crystal up for sale. You won't sell it right away, but when you do sell it, you'll get 430-500 credits for each lot of 100 that sells. That, and watching for random resource mines that your scouts can easily capture, will keep you closer economy-wise to the TEC and Advent.

-- Retro