Promote to variable is simply a quick-access way to make a variable and set it to whatever you’ve dragged off of. I think you’re assuming that “Promote to variable” in some way links the return value to that variable or whatever, but it doesn’t do anything that fancy- it’s just a quality of life feature. It doesn’t do anything you can’t see, and it’s all done when you click that button- not at runtime.
Yes. Promote to variable created a variable and added a set node to your blueprint graph. Assuming that set node fires (at runtime) and the actor it found isn’t deleted, it will be valid. Keep in mind that it also isn’t guaranteed to find anything- if you don’t have an actor of that class in your level when it’s fired, it won’t record anything.
Trying to access the variable before that set node has been fired would also cause an access of none error.