A humidifier will work with using the cold water as the input. It might work better with warm water, but from the problems you're having, I'd just switch it to the cold side and forget about it. FWIW, my AprilAire unit has been using cold water as the feed for years, and works fine.
There are higher trip T&P valves, but they wouldn't be a good idea at your altitude, even if it would fit (they're larger in diameter). They would also void the CSA certification, I think, and if you had problems with the tank, (it could blow up!) your insurance may not pay up, if you were still around to try to collect. http://media.wattswater.com/F-SF.pdf