Does "trade for profit" work in both directions? In other words if I buy from my favorite faction in the system and sell at a profit elsewhere, does this count towards Boom state and INF back home? And since I found a wonderful 'round trip' trade route, can I buy stuff from far away and sell it at a profit back home and also increase INF / Boom chance?
Buying goods from a station doesn't directly affect anything, only selling. But if you find a nice round-trip route, then you may able to get both ends into Boom, which will for
most goods improve the rate at which they generate new supply for you to buy.
Oh, and does actual profit made make any difference, or is it a flat INF+n per trade transaction? And if the latter, do I get more INF by loading my ship with a variety of different goods (each specific good being a different transaction) over loading my ship with just one commodity?
It's complicated and has a bunch of anti-exploit measures. Like a lot of things it was adjusted in 3.3 and then probably fine-tuned a fair bit since, so I don't know if the exact formulae have been calculated. Broadly, more different types of good are good, more different systems the goods are sourced from are good, and both higher profit per tonne and higher tonnage are good.
So long as you're making a profit on the trades they'll probably be okay, though.
I just noticed that USS (like HGEs) are labeled by faction! Like Jane said, once you take the pill, you start to see things... So what does this mean? If I harvest mats from a USS owned by my faction (I assume the wreckage is one of their ships), does this somehow affect the BGS?
USS are by-faction, so the faction's current state will affect what you might find inside them.
If you're just salvaging materials this won't affect the BGS. If you start shooting at ships you find there, then it can. Cargo salvage will have the normal effects if sold. A few of them have scenarios inside which have their own BGS effects.