Well, if it's using all 3 singularities, there's a turn-long gap between their placement, right? What if you plop a positive singularity on it right then?

I know there are notifications when your own singularity expires but not for others

Note that the AI doesn't have infinite singularity slots. Thus a slot used on you is a slot not used on someone else or used on their own systems. Thus there is a fairly big opportunity cost for keeping your system under seige.

Yes that is true, however a counter measure prevents persistent attack on a system.  A player could if so desired keep a singularity active on a system for all of the game.  It stands to reason that the Riftborn being masters of time would have knowledge of both singularity counter measures and how long they would be active for from both sides, this only effects Riftborn vs Riftorn situation.  An interesting thought regarding Riftborn is I wonder if they would know in advance that they would lose a game, and even have the ability to do something about it.  Time manipulation raises some interesting issues.  Could a singularity placed on a players home system slow down their victory condition win possibility.