Wow....not sounding good. It'll be interesting to see how the snake is after it's shed. The heavy breathing and clogged nostrils definitely sounds like a respitory infection....probably got it during brumation. The dark spot does sound like possibely some type of bacterial infection....maybe secondary to the respitory issue or may have also just come from the compromised immune system during brumation. I'd be worried that just letting the snake "self medicate" with tetracycline in the H2O may not be aggressive enough. I'd probably try an injectable med. like Batryl. I've had good luck treating other snakes with respitory problems with it. Not sure where you'd get it though with out a vet....mabye mail order. It does sound like the other things that are being done are good, but I'm worried it may be too little too late.
