Replace resin when output TDS reaches 10-15 ppm.
You always want 0000 ppm, but you may get spot free results with slightly higher tds.
Heat will always speed up a chemical process, so it may accelerate bacterial growth in the RO filter if exposed to extreme heat. Black PVC casing will also accelerate these processes.
The flow coming out of your water source does make a difference, especially at heights 3 stories and above, and with an RO system.