Things like this remind me of how my Father would go at snuck screws, bolts, spark plugs..etc. I'd be to afraid to go forward while he'd pull his sleeves up. I remember the a couple spark plugs wouldn't break free on a old late 40's Willys truck. I gave up knowing they were going to break off. The ole man torqued the plugs a bit tighter, then a bit back counter, back and forth, back and forth for what seemed like forever. Me holding my breath positive they were going to break. A smidge at a time they finally came free. Many times over the years this same trick has worked..... but I'm always nervous as he!! while doing so.

IIRC, your screws looked rough. I don't care for WD40. Better penetrating oils out there like Liquid wrench. Your grips aren't wood correct?
Wonder if you could just submerge the complete grip in fine oil? Thinking out loud....... Someones got a method to get them loose.
Good luck if you decide to attempt. There are markings in/under there.

Charlie-P777