Another minor conundrum
In order for an NPR to accept your claim on a system, it must recognise that you have established a presence. This is defined as populations with a combined EM signature of 500 (about 4m pop with infrastructure).
I'm currently using the Alien Population intelligence as the basis for that, because every time an NPR (or a player) detects a alien population they will update the EM and Thermal signature of that population for the Alien Population record. So when an NPR is considering if your claim is worthy, it will use the latest information it has.
The problem arises when you have a population of the required size, but the NPR hasn't visited the system for a while so it doesn't know the population is large enough. You make a claim and it is immediately rejected on the basis you don't have an established presence. I am considering three options:
1) Leave as is. If the NPR isn't visiting the system, then whether you claim it or not doesn't matter. The downside here is that the next NPR visit might be a colony fleet, which changes the situation.
2) Allow players to pass information on their own colonies to NPRs (and add the same code to NPRs).
3) Use actual population data. This assumes that 2) is happening in the background. The downside to both 2) and 3) is that it assumes races believe each other
Tempted to use 3) as 1) could be frustrating and I think 2) adds an unnecessary step.
Haven't read everything you might have written about claims what considerations there, but maybe instead of making what players and NPC's know a problem, make it so its a feature? I can imagine not knowing if a claim is valid could lead to a lot of emergent gameplay.
So to start with, make it so they do not have to react immediately? Basically, they can be like yup, we are aware of the claim you made and the reasons you provided. Then they have to decide how to react to said claim, you already discussed what makes it valid and how they think of it when they have all knowledge they need at hand so I wont' get into that, what happens first is more interesting to me. First they have to evaluate the veracity of said claim. This can happen several ways. If you got really good relations, they like and trust you a lot and you have some fantastic diplomatic teams, they will accept your word no question asked. If you don't maybe they got some great intelligence on you then maybe they already know it, and thus also accept the facts of the claim. Maybe it will trigger a response from them to go check it out with a ship. Maybe they will ask to be allowed to or maybe not. Maybe they will ask to have some information send, or have an observer team check it out. And you could have them be more likely to accept the fact without question if you do stuff like solution #1 and send info along from the beginning.
This all leads to players having to consider how much info they are willing to part with and consider that versus how likely an NPC will accept the claim without issue. And do note, there is two stages to accepting the claims; accepting the facts, and accepting the claim. The facts are what is what the problem is right now, how the NPC (and players too!) knows whatever there actually is a population there. A player or npc could lie... Once the facts are established, the question is of course, if the fact matters, maybe there is actually a population there, but the system is neighboring the capital system, so the NPC don't want you there no matter what and rejects any claim whatsoever.