Hi there, my thoughts:

1. Trade routes are interesting. Your kingdom will automatically trade within itself and between provinces, provided you have the right improvements. Trade will occur with other factions when you have at least a trade agreement and the proper improvements to allow for trade. Lastly your merchants don't do much other than gaining you florins per turn based on the resource they're sitting on, how skilled they are, and how far they are from your capital. The proper buildings to enable commerce are pretty much the same as in MTW; ports, roads, merchant buildings, etc. Build those, get your trade agreements, and manage your merchants and you'll get some pretty hefty incomes.

2. Sorry, I don't recall if they provide a bonus to order when in the city, but there is the chance that they will get some positive ancilliaries when in a city with church improvements. The best thing you can really do with your priests is, in my opinion, 1. send them off to non-catholic lands to convert the population and as a corollary stir up trouble for the province owner, and 2. burn heretics. Personally, I only use mine for 1. and use my assassins on the heretics as fodder. Just make a big group of them, say 10+, and keep them moving!

3. Be careful with princesses. Do NOT accept marriages from low charm princesses into your family because the odds are very high you will get really bad traits or ancils on your generals from them. Only accept high charm princesses. As for your princess, she's basically a diplomat with sex appeal. You can marry her off to get alliances with other factions, but in practice those alliances based on marriage don't always hold up well.

The only other thing I can think of is that right now you should make darn sure you have the v1.1 patch AND Carl's Problemfixer installed. The game has a number of problems, Carl's patch on top of v1.1 does an outstanding job of putting a bandaid on some of them but it can't handle them all.

Enjoy and good luck!