Brew Screen / Boiler Valve Issues-- what to do when its' stuck?

Recently the brew screen (screen attached to the bottom of the boiler unit screwed into the mushroom boiler valve) ended up in my portafilter. I thought, "no biggie-- just screw it back in." Well, that isn't working-- screw has somehow become too short (though it doesn't appear to be stripped or broken). Ideas? Here are mine, would love some feedback.

1. Maybe it IS slightly stripped and not catching.

2. Maybe there is an issue with my mushroom boiler valve-- went up into the machine?

2a. I cannot unscrew my mushroom boiler valve for the life of me-- tried many different times with various tools.

Suggestions before I start aimlessly buying pieces and trying to replace things? Much appreciated! ~ Stephen