I have a Multiple Entry Non-Imm O issued from an honorary consulate in the States on the basis that I work outside the country and use Thailand as a home base. The 1-year visa expires in January, after which I should have 90 more days if I enter right before it expires, yes?

I'm not sure if I understood completely before, but is there anyway to have this visa extended or re-issued without travelling to my home country?

I'm a US citizen. I will be spending most of December in South Korea. I'm not under the impression the Thai embassy in Korea is particularly generous, unless this was a standard thing.

Also, will this visa and 1 other Thai Tourist visa in my passport present a problem if I need to apply for a Tourist visa in Vientiane or elsewhere in March/April? (I have several more Tourist visas in an old passport if they actually check their records.)

Any help greatly appreciated.

Tourist visas should not be a problem - but another multi O visa for "home base" reason is probably unlikely. Surly not in the immediate area.

Yes you will get another 90 day stamp on a valid visa entry.