Hi, I am reading around 10V at the reel motor (it depends on motor temperature, amount of tape on the take up reel, etc..., also because most probably the internal resistance of the motor is varying during use). I used a 47 Ohm resistor in parallel with existing 30 Ohm resistor, soldering two wires directly on the 30 Ohm resistor pins on the top side of the board, without disassembly anything. I, too, was worried for the transistor dedicated to play (if I well remember the other one is also used during FF or RW, so it already supports the full amperage at the 13V for FF/REW, so no worries for this one). However, the temperature didn't rise to risky levels (I checked with the finger, I have to confess). However, if you are worried, you may easily install a small heat radiator on the transistor. The deck is still running perfectly fine. After a 3 weeks holyday, I turned it on and the reel motor started immediately without any hesitation.
Regarding the idler wheel, there is a felt shim that creates a certain amount of friction to allow the idler to move and position correctly on the take up / supply reels when tape drive direction changes, so it is normal to see some friction. My suspect is that, if you replaced the idler pulley hub drive belt, it may be a little too tight and cause excessive friction on the idler pulley drive axle. Try delicately to loose it by stretching it with your fingers. Or the friction on the take up reel brake may be too much. If everything is ok, the idler pulley must rotate with no slip with the driving hub nor the take up pulley. Instead, slipping must be present between the two parts of the take up reel (the two sections between which the felt brake is present).
Hope this helps... Warmest regards....