See if you can grab a needle nosed vice grips to remove that middle screw, or get a stripped screw extractor set like this (you're gonna need it anyway):
Click here for link to screw extractor set
Home Depot/Lowes/Menards/Ace Hardware SHOULD have something like this, but not sure if it's gonna be in stock. If you can get that screw out, you very well may just be able to pull that next movable plate out without having to resort to the jack. The link in my signature goes to a thread where I showed the disassembly of that style of cleanout plug. These aren't difficult at all to disassemble, except as you've found out, the first time it's done. The thread sealant, shown in the 5th pic, is likely what caused you to strip the screw.
The plastic tab that seems to be jamming your plug in there is clearly shown in multiple pics at that link. I was incorrect earlier, you can see that single center screw is holding the metal plate to the bottom piece of the plug, but the part that seems to be stuck is below that, and held to the bottom piece of the plug with 6 additional screws. That's it, that's the ball game, I believe. Get those 7 screws removed, and you should be able to remove it from the topside with something like a radiator hose removal pick or other hooked tool.
At the least, you need to remove your stripped screw. Once that's done, the other 6 screws had no sealant on them in my case, so you may be able to simply lift that right out, or be able to get a screwdriver in there for leverage or something easier and less likely to damage stuff.
DO NOT lose the two white pieces, and the springs that they sit on. I haven't seen them for sale without buying a whole new plug, and currently running $165 EACH for your boat.
To be quite honest, I don't know what that plastic tab does. Seems to align and place those springs and white bits with a friction fit and just enough pressure to keep the white bits in the molded spots provided. IF you end up having to buy a new plug, I'd be willing to experiment with the (potentially) damaged one, but I'm hoping for your sake it doesn't come to fruition.