From Spindleweave's letter, about there being gaps in his knowledge, I take it to mean that there will be many locations added to this map in the future as we encounter them. Is that a correct assumption? Certainly Khare must have a lot of trading partners along the river and up and down the coast.
There are a few important things to note. Spindleweave's knowledge of the area is incomplete, and she is providing what she knows. As such, it is a safe bet that there are things on the map that have "yet to be discovered".
Should players start by marking on the map where their characters came from, and we flesh out the world that way?
This is something that we will keep in mind, but do not generally recommend that players do this in a "serious way". Because the map is not marked, that doesn't mean that there isn't anything there. The location that players select for their headcanon might already have something there. Does that mean that places cannot
become canon in the future? No. Only that right now they aren't in a meaningful 'put-it-on-a-map' kind of way.
Obviously this is just for illustrative purposes, but could players decide to establish, say, a small camp north of that cluster of buildings near the fork (marked with the larger, red circle) as both where their story starts, and a location to be visited by other players in the future?
This is
conceivable though unlikely. Unless that character is remaining at that site, then any number of things could happen to it. The people living there might be forced away, the camp could be razed, etc.
Purely on a logistical level, this degree of map updating and territory claiming is prohibitive. Territories will change over time and there will be new locations discovered and added, but these will be added canonically through the story and as everyone's story progresses. Keeping track of every location that everyone is from and has travelled to is not something that we will be doing, and broadly doesn't matter all that much at a "meaningful out of character world building" level.
The map should serve as a broad outline of where the characters are, but it is not the end-all and be-all of what does and doesn't exist.
Hope this helps!