What are the obstacles to just assembling the starting seven dwarves as if they were a migrant wave (and possibly replacing their skills)?
Probably the reasoning behind not doing it it mainly because the "seven ideally chosen dwarves at embark" thing is a tested formula for dwarf fortress, and that adding historical figures to the mix results in a bit of ugliness. If you're just replacing their skillsets, then why bother making them historical at all? If you aren't replacing them then how does it work? Do you do searches through historical populations for candidates or what? Would you need a bunch of new screens for looking at entity poulations filtered by skill/social attributes/age/availability? It is one of those things that on the surface is totally possible, but for Toady to do it now would set the release back a month, because doing it
well would take a long time, because it would involve setting up all the embark scenario stuff.
It is the same problem that has been mentioned about the player in adventure mode - they don't have any history. There are potential fixes for this, but all of them are kinda bi picture things that would take a long time, and like anything with DF of substance, it needs to be scheduled a chunk of time for Toady to code it.
As things stand, the starting seven aren't a complete lie, because your civ has plenty of non-historical dwarves anyway. I'm looking forward to having the fully fleshed-out embark scenarios; especially getting a band together in adventure mode and then embarking that way.