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Doing the stucco on the outside will help a lot with the insulation or currently lack there of.
There is not other way to insulate from the inside.
Your house being 50 years old, your outside walls are only 2 X 4 instead of the current 2 X 6. Also back than they weren't all that crazy on insulating houses properly.
No matter what you do, the room above the garage will always be colder.
Spray foaming the ceiling WILL help but will not completely solve it.
I would consider putting in a gas heater in the garage and running it at a low temp so it doesn't only heat your toys in the garage but also help with the room above.
I would call around for more quotes and see if you can get a better price on the spray foaming.
I do know that it is not cheap. There is also the "health/good for your" aspect of it. It has been said that there are studies that the spray foam chemicals are not good for us and can cause health issues.
Most companies do not tell you this but you SHOULD NOT stay in the residence for at least 24 hours of the spray foam being applied.
As for the duct work.
I have small inspection camera that you can borrow if you want to look into your ducts. Hooks up to your mobile device and you can view/save picture and videos.
I have fishing sticks/rod as well as this is the line of work I do. You are welcome to borrow.
Investigate with a FERRET - rechargeable wireless inspection camera
If you do stucco on the outside you need to do it all in one shot.
(I am pretty sure you mean room by room and wife at the cottage if you renovated from the inside)