To change properties of the mesh needs it needs to be a UPROPERTY with the VisibleDefaultsOnly flag.
UPROPERTY(VisibleDefaultsOnly, Category = "Components")
Quite often you will have to re-create any blueprints that are already based on the C++ class if the constructor changes, especially when it comes to creating/removing components or changing the attachment order of things. Certain properties get serialized and it’s very hard to change them afterwards. Blueprint unfortunately gets full of junk data over time that’s hard to clear up, and a lot of these issues can be fixed by just recreating it.
You will be able to set the scale in blueprint, but you won’t be able to set the transform/rotation as the root component isn’t relative to anything as such.
The actors transform/rotation is the transform of the root component - so it makes sense that you can’t set it to something relative in the blueprint editor. If you need to set a relative transform for the mesh, then you’ll need to create a scene component at the root as you did before. If you want to expose the properties you’ll need to make another UPROPERTY for the scene component.
My gut feeling is that the blueprint needed recreating and the mesh wasn’t attached to the scene component properly.