About knowledge... I can't say if they are still being generated off site, but I can say that PLAYER generated knowledge in one fort can make its way to another. I resurrected a conquered civ (king was crowned off-site in a goblin conquered old fortress), so my site was both 1) the only site owned exclusively by my civ, and 2) NOT the mountainhome (king off-site, bugged 0 exported wealth/offerings prevent noble progression). I also started a huge library and eventually worked up to 12-15 books written on site (caravan never brought any).
After 6 years, I started a new fort. The first caravan brings a book written in the early days of my previous fort. It was a book about a book about traveling to my last fort, written in the same fort, by a dwarf with a nickname (so definitely written on site). It was a copy, naturally, and I had been selling these to the caravan. I believe caravan goods are destroyed upon leaving the map, and they were bound with rocks (and I only used metal for book-bindings).
My point is that a book written in a player-generated fort was having copies of itself sold to other forts in the same world. The next caravan brought about 5 books, half of which I remembered having been authored in my fort as well. In a few years of game time, I might unretire that fort and check if any new books had been written.
As far as migrants being actual figures... yes. Yes they are. As noted, the starting 7 and first couple of waves -can- be generated out of thin air if no one suitable is around. I've gotten dwarves from previous forts in the first two waves though. In the same fort I mentioned above, 3 of the dwarves within the first migrant wave had already had moods in my last fortress, and a few others are recognized from labor layouts and/or had nicknames assigned to them. My third wave was actually entirely made up of scholars from my old fort. A mixture of freshly drafted migrants and some military dwarves I drafted into the library (to see if their higher student/teacher levels would help them learn quicker). I set up a library and they even wrote 7 new books that season.
It also seemed like their locations assignments remained in-take. Despite not being assigned to a location on-site, they actually arrived as historians and scholars with the purple icons. I even got a tavern-keeper to migrate. She just showed up already as a tavern-keeper, but wouldn't serve wine until I reassigned her to the new tavern.
And in that first fortress, ever single migrant was "vengeful for joining an existing conflict." Considering my civ was still at war with the goblins, I assumed that is where the thought can from. They likely weren't pre-generated goons either, and were either refugees or still living in that goblin conquered site (where the king resides).
Oh yeah, and one of the human dancers visiting my tavern got promoted to a "lady consort" on site. I suppose her husband inherited a title, and she got a letter in the mail informing her of her new blinking name-tag. I checked in legends-viewer, and a war was being fought while my fortress was running between some humans and the nearby gobbos. Sites were taken over while I grew plump helmets behind my fortifications. Visitors were shuffling from tavern to tavern too.
I believe caravans and liasons can be blocked by traveling armies, but I've always heard this as second hand (or worse) information. I do think the contents of a caravan are randomized as soon as it is created at your map-edge, though. For no other reason, the old trick of forbidding your wood/cloth/food stocks the week before the caravan arrives will force it to spawn with more goods of that nature. So even if they caravans are tracked across the map, its contents are not.