I'm not really going to get into the technical aspect since I feel neither of us know enough to tell how feasible it is (although I think you're wrong since you do need trackers in order to find at least one other member of the swarm), but this part
If they both aren't online at once, messages won't get delivered. Which is not a big deal for a modern smartphone given that most of them are online close to all of the time.
I just a horrible take. You can't base your business model on "modern phones being online close to all of the time". You can't have random data loss whenever someone goes out of service area, has to turn on airplane mode, runs out of battery, has a software error or just an update or some other kind of temporary downtime? That's not how you design any software, less alone a dependable messaging service. You can't just "stipulate that".