Load-bearing or non-load-bearing, two structural engineers say different

[Vrana]

Do you mean the cellar ceiling underneath? It is a concrete slab.

 

[Workingclasshero]

Even if the roof structure was free-spanning from the beginning, you can bet that the continuous wall right under the roof, which in turn stands on a masonry basement wall, now serves as support for some of the support loads that are easily led down to the basement wall except precisely where TS wants to demolish.
That part of the wall, stiffened by wall panels attached to beams headband and sill, practically functions as a beam for that section of the structure in the ceiling and floor that hangs over the basement garage.

Brace up the garage ceiling so it remains straight. Tear down the wall and be prepared to brace up any sagging under the bottom of the truss. If it sags noticeably, I would install a beam/stiffen the truss to avoid larger movements when the maximum snow load presses on.
Feel free to measure the floor in the room next to the demolished wall when the garage prop is removed to ensure it doesn't start sagging: Even a half-centimeter fall on that floor area will be noticeable.

(I have an almost half-century-old education in construction but have never worked with it in practice)

I wouldn't do it that way.
First assess the condition of the trusses and possibly level the ceiling.
Then make necessary reinforcements based on information from someone who can see the current situation with their own eyes.
THEN, not BEFORE cutting away the wall.

Not like this, in other words...
Good luck.
/Workingclasshero

 

[Markus_snickare]

I would support the rafters resting on the wall you want to remove by placing a cross beam on top of the rafters in the attic and securing it with iron. This way, you transfer the load to the nearest rafters that still rest on the wall that remains. Since you are going to vacuum out the sawdust anyway, it will be relatively easy to arrange.
Even though it says they are free-standing, the requirements have increased over the years. When I've demolished/renovated old houses, there can be really weak rafters in some places that don't meet today's standards. However, yours do look quite substantial, but the low roof pitch means the snow load is large, and the weather has become more unpredictable, so better safe than sorry.
You also have an opinion from a structural engineer, so you're not doing anything wrong in terms of the insurance company, etc.