I think you are getting a bit carried away here.
All you say is that the gun is inconsistent. The corrosion/whatever, wipe it off and be done, for now at least.
The inconsistency could be due to the fact that you have just taken it apart and there may be lube where there should be none or very little.
Also, these guns have one known issue, that is with the piston seal detaching from the piston. Now when this happens it generally sits at the base of the cylinder, just blocking the TP, so you fire, and all seems and sounds OK, but the pellet just stays put.
Perhaps - just perhaps - something related to the piston seal has happened.
For starters I would test it further and see whether it settles down, and if applicable, stops dieseling.
Second, I would take it apart again and check that all is well with the piston seal, and if yes, re-assy with care to the amount of lube and where it's going.
Then, test again.
And obviously as Slugger mentioned, check other potential obvious culprits like the breech seal.
If it's still crazy and you are sure it is not dieseling only then should you worry about anything else, and Slugger's technique seems very well thought out indeed.
**WANTED**: WEBLEY PATRIOT MUZZLE END; Any Diana/Original mod.50 parts, especially OPEN SIGHTS