I guess the larger questions are: what do sites revolve around besides being gathering spots for NPCs? Any location with a market of sorts, or a tavern has a reason for being as there's clearly either industry present/a focal point. Will NPCs be seen "at work"? Hunters never hunt, farmers are never in the fields, etc.
Are there any plans to diversify the human seat of power in a hamlet? Instead of just meadhalls, will there be manors, or other structures? Tangentially related, will we be able to mod the term of description for such structures?
The game doesn't have the robust economy/property framework to justify the presence of many sites for reasons other than "we can build them", even though the economy is coded inside the game. Stones, metals, climate, trade nodes, population and natural features influence the existence of these sites, but some of those factors are currently so abstracted and out of play that we barely notice them. As soon as the pertinent updates roll out (along with the scenario embark) then we'll see more plausible reasons for sites to exist within a certain world. However, that's not going to happen anytime soon.
Same with the NPCs question, it has been stated that the eventual goal is to simulate a living, breathing world just like a world-wide functioning fortress, and DF has taken steps toward that direction, with bandits harassing and stealing money (albeit in a rather hilarious way), soldiers patrolling and adventurers doing their stuff during their "important missions". As you have been reading these FotF, expect to see more of that with the next update. As magic and magical artifacts make their slow and steady appearance, they will surely flip the rulebook regarding power structures, and even if they're not editable in the RAWs I think their random nature would be enough until we can fiddle with them.
(Sorry, I have no idea about the ambush/stealth stuff you mentioned and reading through your posts, it seems you have a better understanding of those features than me. A question definitely better suited for Toady himself)
As for the seats of power, the answer is "yes, no timeline yet" We only have meadhalls at the moment because those are the only meaningful interactions developed yet. With future releases we might expect to have influential churches or guilds that provide a similar function, and possibly a proper graphical representation of the different factions that may vie for power in a single site, but only time will tell.
When the myth update deals with the origins of the races, will there be any sort of like potentially semi-divine "first dwarves" or "golden age humans" like sort of the primogenitors of the particular races, who could get sung about for their role in history/cosmogony in songs and depicted in art and stuff?
The few things that have been revealed so far regarding the Myth Generation seem to imply the possibility of what you're asking about, although Toady hasn't mentioned their gameplay impact yet, and I'm afraid it won't be gamechanging during this pass. It is likely we won't be firsthand witnesses of the world creation, although the described events will surely influence what kind of materials and spoilers we find within a given world, and just like that, we will certainly be able to sing about cosmic eggs, divine serpents, spoiler metals and such, or even find physical evidence of these.