I don't think you've killed your replacement motor.
Don't beat yourself up.
But don't forget this either.
I learned a terrible lesson years ago when I fried my mint Tascam 58 with a careless tiny mistake.
I can't underscore enough for readers of this how important it is to be slow, careful and methodical with work like this...and ALSO be prepared that if you are moving into uncharted territory, working outside of your skill-set, realize you could jack your unit up for good. You could also kill yourself...milliamperes can stop your heart.
But...truly, don't beat yourself up...it does you no good at all...won't fix anything. You've got what you've got now and you should be realistic it may be tough to fix so accept that and let's move forward okay?
When you powered it up with solder lands 19 & 20 bridged on the MECH JOINT PCB, what did you see/smell/hear? Where did smoke come from etc?
Let me look at the schematics for the JOINT PCB and see if I can point us in the right direction.
You've learned a lesson. Use it for good moving forward. Get some better light.
BTW "tracer" should just be "trace". Solder traces connect solder "lands" or "pads" where the components or wires are mounted.