The absolute best way to remove and install most jewels in Parker Vacumatics and 51's is actually very simple. Get a hot glue gun and put a generous drop on it. After it hardens, you can turn the glue blob and the jewel comes out easily. Peel off when done. I've done this tons of times with no ill effects to jewels. Anything stubborn just requires a 5 minute soak in water before turning. To date, I've never had it not work. Hope this helps someone, since it beats all other methods I've tried by a long shot.
I am thinking of trying using sealing wax to hold a Parker 75 tassie so that I can unscrew it. I have gotten some off using a rubber band etc. for gripping but some tassies are more resistant to unscrewing.