The power valve theory sounds plausible to me.
Black smoke, soot colored plugs and an eye burning exhaust probably DID in fact occur.
My guess is you were so focused on "other factors" you failed to mention these critical symptoms early on in the thread. It was in fact sometime later that you mentioned the soot colored plugs, all most as an "after thought".
Oil dilution to a very rich condition does fit the symptoms. Severe
oil dilution would lead to other probelms as well. Anything that requires lubicration would be affected. Like,,,,,valves?
REAL lucky you didn't throw a rod!
Ernie