All the datasette schematics checked show that 6.8V is "motor on" (motor supply only).
The P4 line is driven low by switch that closes on pressing any key that requires the motor to spin.
Yes this is confusing me , There should be no need to press play to active Q2 as rewind and forward should work all the time.
Confusing me too actually! I don't know enough on the C64, sure some people out here do though.
As the only way to supply motor is through Q1, the P4 line must be pulled low also for REW/FF.
Then I'm not sure how this is handled once you have cued the tape and LOAD or SAVE a program or do you just have to: cue, pause, LOAD, un-pause, wait... ?
Your problem could very well come from the MPU not detecting the Datasette (therefore no motor power switching), however it is rather unlikely that the CPU itself will stop the P4 line from changing state as all you have there to oppose to that is a pull-up resistor (RP5) and a logic input (if that was destroyed you'd have heat or other problems driving P4 low).
If you can get P4 to go low (you can simply force it low for experimentation using a ground strap) and you have checked that the motor power switching works, if you still have no normal operation that leaves you with the possibility of a defective MPU.
I suggest you open a Datasette and connect it to the C64 check it's cable continuity including ground and shielding.
When Play is pressed P4 must go low, first problem to solve.
Very often the supple contacts on tape mechanisms develop a black oxidation giving high contact resistance. (Does P4 decrease at all?)
A contact spray and strip of paper as an abrasive is usually enough to do the trick.