Congrats,post pics!
PTO shafts do get rusty even if they are new sitting in the stock yard... I would spray some "kano" or similar oil onto the coupler and let it set over night... just my two cents...
Is the end that attaches to the tractor rusted just as bad? Maybe you could take it off from that side easier, and that would make it easier to work on? At least you could get the mower out in the open.