You are on the right track, and it just about has to be PROBABLY a vacuum leak or a fuel supply problem, as you say, floats or a restrcition/ problem in the fuel line/ pump/ filter/ delivery system
You mentioned "PCV is not blocked." Is it hooked up? You might TRY blocking it. The valve might be broken/ stuck/ leaking
The choke vacuum you mentioned sounds like the choke pull off. If so, this IS manifold vacuum and WILL affect idle, --but not to the extent you mention.
How about timing, where is it set?
If you can get it to idle, you can squirt a LOT of different "stuff" around the carb base, intake area to listen for the engine change. Even WD-40'll work.