Why are you trying to remove the bedknife from the sole plate? You have a very good chance of stripping the thread in the soleplate, and you can't achieve anything useful by removing it anyway. The bedknife has to be properly fastened to the soleplate to be able to grind it. Since it appears you don't have a suitable clamping facility to properly remove the bedknife, you don't have a facility to install it either.
Normal practice is to deliver the complete soleplate (with bedknife attached) and reel to a reputable, specialised machine shop and have them sharpened together. If the bedknife needs to be replaced, they will do it.
