I have been using tankless heater for over a year with my Mytee Escape and it gets about 120 degrees...the pump can take up to 140...I have not had any problems with it to date, I have pics and web link somewhere on TMF of it.
I have only had to deal with the pump once on the 2 vac Escape and it had frozen and cracked the seals, had to replace that one
When I first got my 4 vac ETM I had to prime it and it still wouldn't work...took me a few to realize the chemical jug was empty and needed water, also the prime tube...
On your pressure not working I would make sure you have water or rinse in the chemical injection jug
Prime it and use the vacs on the primer hose.
I would make sure your vac sensors are not stuck on the pump out problem.