So funnily enough after I sent you my suggestions I had a similar situation. I dig up two old Innokin U-Cans that I hadn’t seen in … I don’t know when. Old enough that the smell when I opened it up the juice it was… enough to make my face change color.
Anyway I could open them up and get the juice reservoir open but the part that covered the needle wasn’t going to budge. Not even my wife and her famous monkey grip could do the job.
I soaked them in Dawn and white vinegar for about two hours. One of them came open. Had to use that round grippy kitchen jar thing and the round end of the ceramic tweezers and a lot of muscle but it came open.
The other still wouldn’t budge.
Put them through the dishwasher. Then again with the rubber grippy and the bulbous side of the tweezers and that got the second one.
So give those things a try I guess. Good luck!