Both cash and XP are determined by length of the route. Th elonger the better. But XP are indifferent to type of good, so if you're trading for cash, go after the longest route with high-level goods. If you're trading for XP, then go for the longest route period.
Of course this means that the best route is a long lux-narc route, since it provides both high XP and high cash (+ high risk from hunters !)
Often it pays in the long run to use less-tan-perfect routes, since this makes it more difficult for hunters to locate you ...
YES, starting by trading adjacent ports to increase relations is worth while. it costs you some turns in the beginning, but pays off fast. Remember that the more holds you have, the more your relations increase per turn, so don't go trading in your pod to increase relations
While you trade for relations - just accept the prices offered and your relations will increase automatically.