Thank you all so much. You guys are so helpful.
So I got the seat and seat post out! Bent the ears back open a little bit with a screwdriver and dripped some oil along the post, then was able to twist it out. The post isn't scratched or gouged.
I am 99% sure that the post fits the tube, and I strongly suspect it's the stock post. The saddle has the same branding as on the frame, so I'm sure they came together. I can't prove the post wasn't changed at some point, but a visual inspection hints that it's about the same age as the bike. Plus, I know people rarely ever change parts on these low-end bikes. And finally, after the oil kinda got spread around the inside of the seat tube, the post seems to telescope into the tube normally - snug, but not slipping all over the place. This makes me happy, cuz I'd like to reuse the post.
Next I'll clean up the rust on the post and inside the seat tube, regrease, and then see how things fit with a bolt. Will keep an eye on whether the ears want to smoosh inwards again under force. I didn't see any cracks forming at the joint, but I'll try to keep tabs on that.
I am so excited to work on this bike. I like to overhaul all the bearings, true wheels, etc. That will be no problem, it's just that I have these little pockets of ignorance on stuff like this which I've never encountered before. You guys are the best. The rider will be so happy.