First off, it looks like you are doing some great work. Secondly, are you using a piston stop or just a strap wrench? I’ve been able to remove some stuck ass magnetos with the motor off using a piston stop and my fat ass to hold the motor in place. You could also try pulling the jug and cutting some pieces of wood to brace under the piston. You can cut them long so you can use those to hold things down while you work it.
Note: supporting from the under side puts a lot of strain on the piston skirt. Not ideal, but also an option.