gashot-waterplumbing
my water heater (gas Rheem MODEL NO.42VP75FW) keeps on shutting off 
this is the error code it shows
it means the maximum amount of ignations attempt have been made and the system has went into lockdown mode
when i turn it off, wait a little, and turn it back on the system works for around 10 minutes (heating up the water) before locking up again.
what am i supossed to do ?
I have the same water heater and those knock noises are the flow valve actuators turning on and off...completely normal.
I've been looking into a heat pump replacement.
You said "However, when the heat pump is in heating mode, the exiting refrigerant line is actually colder, not hotter, than when it enters. As a result, you cannot heat water during the winter."
That will depend on the location of the internal connections. The HP models I have seen with a desuperheater ( hot water generator ) place the HWG after the compressor before the reversing valve. That means the pump always runs forward, the HWG always has hot refrigerant to use and it works in summer and in winter. The reversing valve does just that - in summer it sends the leftover heat to the outside and extracts heat (cools) the inside. In winter it sends the heat to the inside and extracts heat from the outside. 
Best Answer
After calling Rheem customer service, I was told I must replace the entire control unit which controls the igniter.