Trying to remove a stuck butt is fraught with dangers. The rust builds up along the length of the retaining bolt. This jams up solid against the walls of the butt, making it almost impossible to turn the bolt. In the most severe of cases the pressure of the rust against the wood of the butt can end up in splits to the timber. whatever you try you could end up with a destroyed butt. I have heard of people taking the drastic action of actually delliberately splitting the butt to remove it and gain access to the retaining bolt. The butt was then glued together and the split made good.
I restored a 1910 SMLE last year, and I had exactly the same problem. Only in my case the butt had already started to split in the area of the bolt. In the end the damage proved to be so great that I had to source a new (old) butt. Such are the pleasures of gun restoration!