would wearing a spandex balaclava help reduce the risk of the mask rotting?
I believe so.
Anything that acts as a barrier between sweat/oil is beneficial 
Oh, yeah. I do that, myself
I always wear a balaclava when I wear a mask for Halloween night, or any event. Even just for costume shots at home I’ll wear one.
I always wear one, it also helps me a bit with the Castle stretch, but I suggest one made of cotton, spandex or polyester would make you sweat even more…
Good point Kevin 