Ah yes, tons of chlorine, first via that specialty what-cha-call-it spray and then, in a vain attempt to deal with staining, straight chlorine bleach. But I understand "straight" really means just 5% chlorine as household chemicals go. It helped. Lots of scrubbing, with a hand brush and a deck brush.
Changing the ventilation isn't a go. This is a log home, and so putting in a ventilation fan means whacking out a chunk of 6" thick log. That means the wiring is a big time hassle too. There's already a good sized window right there, which explains the lack of a fan (it's one or the other as the code says). I thought about painting it, but decided that would look cheesy. I looked for finished paneling but didn't see any that inspired me.
It took >25 years to get to this state, and it is what it is minus what I can do; once dry I will close the pores with the Felspar varnish. The entire room is wood siding with those 1/2" bevel gaps. I can't make it look like nothing has happened (there is some remaining stain) but it will look a lot better and it will be healthier. Our cat, Sirius, is supervising in between trips outside.