Does the relative motion of the moving parts not affect the orbit/path to L2
No. This is conservation of momentum, if you want to.look it up. Basically, springs latches and extensions could.open and close, motors could turn, telescopic parts could move, radio could turn on its gimbal to lock onto earth, and none of this would change the telescope's path through space.
Why? Same reason that if you jump off a diving board, then (ignoring air friction which doesn't really exist in space), you can move your hands,legs,body, you can spin and extend or curl up - and your centre of mass (centre point of your body for gravitational purposes) will still follow the exact same path to the water it would have done anyway. You might land the other way up, but you'll follow the same path.
Basically if a latch opened on James Webb, one end moved one way, the other end moved the other way, no net change overall. If a motor spin its spindle, the things attached to the spindle turned one way, the motor housing turned the other way, no net change either.
The only change would be if it altered the direction it was pointing in space, not its position or path. Then, when you fire a thruster, it might be in the wrong direction. But that can be addressed quite easily with care, and it's clear they did.
and so could it have been done at either time?
Yes, it could. It could have done it any time outside earths atmosphere. But there's no advantage to waiting, and as other answers say, there are disadvantages - space can be a harsh environment.
Better to unfold sooner, less passage of time for anything to develop an issue if it was going to.