That just popped up a day or 2 ago but I don't understand your "bending the stem to get around the sponge" message. Is it the one in the pic, if so it looks pretty normal I don't see anything obstructing it's growth. LOL I've read your first post like 5 times are you asking should you remove the sponge from the plastic it sits in? That answer would be no, you don't want to move them to early, I typically don't move them to Solo cups until they actually have a true set of leaves, the first 2 you see are the called Cotyledon (had to look up spelling) they will eventually turn yellow and die off, those aren't the "first leaves" I'm speaking of, I'd say you got at least a week to let them mature before you need to transplant
Don't be to worried about the roots hitting air, I recently pupped a seedling upside down, I got excited one morning because it was popping up, came home that night and noticed it was the dam tap root and ripped into a piece of rock wool... gently... pulled it out put it back in and it is on week 6 of veg now!!