I'm betting the crank seals are shot, running lean is often an air leak which can be a crankshaft seal, I got a free Echo weedeater from work that way, the seal was just pushed out somehow, and needed a regulator diaphragm, pushed the seal in and replaced the diaphragm and have put many gallons of fuel through it...
Sounds like you covered most other options, unless one of the sealing surfaces on the jug are bad it sounds like crank seals to me, if not aviation seal or indian head sealant could be your friend...