OK, I'll take the bait...
Here are a few things that I'd consider.
The lower explosive limit (lowest concentration by percent volume needed to form an explosive or flammable mixture) is pretty low for paint thinner, for example. It's less than 1% (gasoline is 1.4%). Keep in mind now we're not talking 1 gallon of paint thinner to 100 gallons of air. It's 1 gallon of pure thinner vapors per 100 gallons of air.
There are only 14 gases lighter than air. I not able to find a single solvent that had a vapor density lower than air. This means that any solvent vapors (that I know of) are heavier than air and will have a tendency to accumulate in higher concentrations at the lowest point of the room, beit the floor or otherwise. This includes right around the pilot light for a gas/propane water heater.
Do I think you're being overly paranoid? Absolutely not. I'm not trying to scare the bejesus out of anybody, just to get people to think. Although, people who don't think make for fantastic job security for those in the fire and EMS service-in fact, they make our best repeat customers! :lol:
I'd look into a few ideas. Moving the waterheater? Put it on the other side of the wall? Put it up higher? Maybe build a box around it that's sealed off from that the inside of the house? Granted then it'd have to pull air from somewhere else, either outside or elsewhere in the house. I even thought about a box built around the water heater under slight positive pressure compared to the garage perhaps from a fan bringing in outside air, but then I'd worry about the positive pressure air blowing out the pilot as it exits through the "chimney" pipe.
Certainly whenever it's warm enough out, I'd try to create come kind of cross ventillation that pushes solvent vapors away from the waterheater. I'd also stick with finishes that aren't sprayed in the months where outside ventillation is several frostbitten toes out of the question. I can't imagine that a brush or wipe on finish creates less vapors than a spray. Other than that I can't think of any cheaper/easier solutions. Maybe somebody else here will chime in?
BTW, if I'm wrong on anything above, don't be afraid to call me out on it. I'd rather that than be spreading misinformation.