Thanks very much T-mar for helping with the photos, and for everyone's advice. After I finally got a seat post in the correct size, I got to work figuring this out. The first problem was the seat post bolt - the nut was only able to screw off half way before getting stuck. The bolt was a carriage bolt, so couldn't get any purchase on unscrewing it further. Luckily the lugs were spread out far enough, and after some delicate hacksaw work (and a couple unfortunate but minor paint losses...) managed to saw the bolt in half. Then it was a matter of carefully prying out the cinch slot opening, doing it as slowly and evenly as possible. I had to hit the inside of the tube with a half-bastard file to even it out and get the last bit of width out. There was a bit of initial stiffness in inserting the new (greased!) seatpost, in the first inch or so, but after that it went in smoothly - so as I suspected the sizing was right, and the tube was likely crimped at the top. The one remaining problem was the brake hanger, which was still fitted to the size of the crimped lugs. I carefully spread these out using some needlenoses - bit of a hack job but it worked well enough. A new 5/16" hexbolt and nut are doing duty holding it all together.

I've put a picture on my profile/ albums if anyone wants to see how it looks now. I want to go back and even out and deburr the sides of the slot, as well as clean up the new seatpost (which has some marks from the previous owner), but otherwise it's now an anxiety-free ride.

Regarding bike ID, after research from past posts from the resident Sekine experts on this forum, I'm 99% sure it's an SHS - Titlist derailleurs, Fingertip shifters, Tourney brakes, Shimano hubs, alloy Araya rims, SR crank, Sakae stem and bars. I've put a photo up for how it looked when I picked it up. The peddles are a replacement, some touring Shimano ones from the 80s, and the brake levers might be later as well. The seatpost that was in there was unmarked, but it was a straight post with a separate clamp for the saddle. It's in pretty good nick all-told. Debating doing some upgrades to some period Dura Ace, but that's long term and it's a great ride as it is. Thanks again for everyone's help!