The seat post is probably the first issue to clear up. I'd be interested in hearing from others (such as Gugie) about what can be done to get the seat lug closer to round. Honing or reaming it to a rounder state isn't actually fixing the problem. I had a frame builder do a bit of work on a beloved frame, and he decided he'd ream it so it was rounder, and managed to change it from a 27.2mm opening to a 27.4mm opening. It's not easy to find a 27.4mm post nowadays! Plus, what is the effect of thinning the wall of seat tube?
As far as components.. I had a '74 Gran Sport, and I changed a lot of parts! Many weren't that great when they were new, imho. The Stronglight crank is a beauty, and I'd hate to see it retired. Granted, finding the proper removal tool might take some time and money.
Not sure how good the B15 is/was, but I'd start with some proofide. After that, maybe a little neetsfoot oil? I'm not sure what saddle soap is supposed to do, but it sounds like it would be removing some of the oils instead of adding them back. I've never used saddle soap on bike saddles, but have used it to clean leather chamois in wool shorts, a long time ago.
and for fun, a shot of my old Gran Sport, circa.. 1980, I think.
Steve in Peoria