So. You guys are not going to believe this, but...
I decided to give things one more try before taking that front plate off. I loosened but did not remove the two plastic screws, braced my hands against the head, and pushed the lens back and down firmly with both thumbs...and it did, in fact, pop right out. Maybe all the fussing I did with it last weekend loosened up the spring, or maybe I was just able to picture the motion I needed better after learning more about it. Either way, thanks to everyone who chimed in! I've got the new lens and board in, and it seems to work just fine with my negatives.