With the motor off and the drive clutch and triggers disengaged you should be able to move it around easily.
With the motor off and pressing the drive clutch it should be very hard to move around and the tires scrape or drag instead of roll.
With the motor off and pressing the drive clutch and one trigger it should be hard to move and probably pivot a little to the non-engaged trigger side because that side has traction.
With the motor off and pressing the drive clutch and both triggers it should be easy to move straight or turn easily to either side.
With the engine on, the drive clutch engaged it should go straight.
With the engine on, the drive clutch engaged and one trigger pressed it should turn to the trigger side.
With the engine on, the drive clutch engaged and both triggers pressed it should not move forward.
Your problems acts two problems.
1. Like the friction disk is engaging when it should not be engaged. When the drive clutch is not engaged the friction disk should not touch the drive plate.
2. Assembly 42, subassembly 42J should be active and causing the wheel to turn with the axle when the clutch is engaged. When a trigger is pressed the 42J for that trigger should disengage that wheel from the axle. That will cause the machine to drive into the trigger side. Even if it does not “drive” into that side you should be able to turn it into that side very easily because that wheel should be freewheeling with the trigger press. If these things do not happen then suspect 42J. The other related parts around there I think are pinned or keyed so checked those are ok (10, 19)
Flip it on it’s bucket, take off the bottom cover and see if the friction disk is touching the drive plate. You could also watch what’s happening in there when you hold down (tape) a trigger. Try rotating the wheels when the clutch is engaged and not engaged. Do the same thing with a trigger engaged and not engaged. Look at what’s happening in there and try moving the wheels.