To connect toilets that are designed to be connected to the fresh water system to the fresh water system. It's NOT ok to connect toilets that are designed to use sea water to the fresh water system.

If in doubt, read the installation instructions! If you don't have 'em, download 'em from the mfr's website.

As for the rest of your questions...you can read all the about the Lectra/San (latest incarnation has been renamed the ElectroScan) and its kinfolk here: http://www.raritaneng.com/products/waste_treatment/index.html Now you know what they look like.

Yes...siphon breakers can fail. However, without proper maintenance and repair, so can thru-hulls, sails, running rigging, standing rigging, electronics, etc, etc.... So that's hardly a reason to avoid toilets that are designed to use your fresh water.

You said you're not sure how the toilet can be "perfectly isolated" from the fresh water source. That's a bit of hyperbole from Jabsco. But along with siphon breaks and backflow preventers, the mere fact that the flush water is under pressure also helps to protect the fresh water supply.

I'm not a hydrologics expert...but I've been at this long enough to know that US marine toilet mfrs wouldn't even THINK of marketing toilets designed to tap into the potable water system if there were any real danger of a problem that could get 'em sued for ANY reason. So I'm back to my first answer: NO toilets that's designed to use SEA WATER should ever be connected to the potable water system.

"But we don't drink the water from our tank(s)" doesn't wash...'cuz you ingest a lot more of it that you think you do. You wash your hands in it...then pick up a sandwich. You rinse out a cup in the sink...THEN fill it from your bottled water. You wash dishes in it...bathe in it... So just 'cuz you don't actually drink it, don't think you don't drink it. So you need to protect it.