If they try the kind of shit you're saying here then they're completely stupid. We control the Relay by virtue of the fact that we have a goddamned fleet and fortifications there - they ceded the Relays in question to the Rachni, we took them from the Rachni, and we're holding them. Them trying to make a claim would be laughable even by the standards of their own constituents, especially in the face of them leaving us to die.
What makes you think most of their constituents have this kind of powerful respect for the right of conquest?
If a hurricane forces most of the population of a city to evacuate, the city doesn't magically become the property of the few people stubborn enough or trapped enough to stay behind.
They can't afford to start a war with us to try to take control of the Relays right now, so they're far better off just recognizing our territorial claims and working with us rather than doing everything in their power to piss us off. It's not like the Asari among the Council won't think to play the long game anyways.
Alternatively, the asari can simply go "hum te hum te hum" and neither acknowledge nor deny our rightful ownership of the relays during the war. What are
we going to do about it, stop fighting the rachni ourselves? The entire (supposed) basis of our claim in the eyes of the galactic public is that we're badass survivor rachni fighters.
If we stop fighting purely because the asari
delay acknowledging our sovereignty over the mass relays we've taken to date, we're going to look incredibly obnoxious and reckless to many people, including some of our own. Like Achilles sulking in his tent, to use one of the few pieces of Earth popular media that actually existed during the rachni wars...
So basically, the Council gets all the benefits of our participation in the war, for the duration of the war, without promising us anything, but without
denying us anything. They're playing the long game, because
of course a body that's 50% asari by weight is playing the long game.
The war is over. Then they start using
salami tactics against us. Do we use our claimed "control" over three or four relays to block civilian shipping? If so, the Council has cause to negotiate access rights with us and crowbar open access to our space. If not, the Council has cause to fight a war after "repeated provocations" wherein we refuse to let civilian ships use 'our' relays.
If we let civilian ships through, do we let warships? Do we let Citadel authorities found new colonies and bases in 'our' clusters? At what point are we willing to confront the much more powerful Council over the issue of use of the relays we claim to hold by right of conquest?
And in the background is the threat of the poorly controlled krogan. If we like right of conquest so much, maybe we can go butt heads with the krogan warlords eager to seize planets of their own...
As far as the quarians go, bad example - they were being punished for creating the geth.
They were
both being punished
and being denied the ability to simply take worlds they want.
The Council does not let people do that.
The two cornerstones of their influence are control of the mass relay network, and the right to apportion planetary resources among various galactic powers. They will not give those up lightly.
If we rejoin it would end up under better terms, like us actually having an embassy, due to being recognized as a fully sovereign nation with our territorial claims intact rather than just some independent colony that only controls a single planet.
I think we can get better terms IF we have a good negotiating position.
I don't think opening with a claim of independence and claiming sovereignty by right of conquest over multiple mass relays and star clusters is a good negotiating position.