If the motor brushes and armature are in working condition and the cage/motor is not jammed and or seized, they will always spin up when 12v is applied to the terminals. Did you check that condition out of the car?
Again, as I'm sure you're aware, if the motors themselves are working they should run on high in the car if you have 12v on the brown with blue tracer at the brown connector - even if the darlington transistor circuit board is compromised.
Regarding the small PCB with the coils that you mention, I haven't heard of anyone having trouble with that and can't say I've ever read a post where someone changes out those components. If you modified the relays in the housing with external diodes, is the polarity correct? You could try refitting the original relays to see if that makes a difference.
The blower can be connected (uninstalled) in the footwell for testing purposes if you support it on a block of wood or some books etc.
My advice would be to remove the passenger blower once more and test the motor itself out of the car.

Might be a good idea to start a new thread rather than continuing this "how-to" one too.