If you can't rely on them, what's the point???

No the don't. The way that the returns are set up, I don't think they would help. They bulkheads are about an inch below the water level. There wouldn't be a siphon to break.

Two of them exhaust straight out from the bulkhead, one has an adjustable arm on it to direct flow some more. I suppose I could use pvc or lockline and bring it up over the waterline and then down again and then drill siphon breaks...

Yeah, having had a few check valves fail on me, I don't trust them anymore either. Indeed, what's the point. They work to a point but I guess eventually the slime buildup inside the pipes gets to be too much to form a seal or maybe the spring starts to corrode.

Another idea with locline is just have the nozzles exhaust near the water surface. This is what I do on my tanks. (And then just have enough flow inside the tank to mix it up.) That way there's no siphon hole to get clogged. On one tank I had vermetid snails and valonia growing over my siphon holes on a weekly basis, I grew weary of constantly scraping them clean every couple of days, so I just pulled the whole manifold out and replaced it with locline outputs.