Has O.Henry made an error in his use of "wot", which I find is first and third person singular. It seems to me that he
should have used third person plural. What would that be?

"Thus does one sister of the plain gold band know another who stands in the enchanted light that shines but once and briefly for each one. By rice and satin bows does mere man become aware of weddings. But bride knoweth bride at the glance of an eye. And between them swiftly passes comfort and meaning in a language that man and widows wot not of.

I think you're correct, wwh, in how O'Henry misconstrued the use of 'to wit.' According to MW Unabridged, 'wot' is strictly used in first and third singular, as you pointed out, and 'wit' would be used for third person plural. I don't recall ever having seen the present participle 'witting' used anywhere!

You're definitely right, Dr Bill. I thinketh this sort of mistake is often madeth with archaic grammar.

The verb wit belongs to special class of verbs in Germanic that while preterit in form are present in meaning. Thus no 3PS in -s (The others survive mainly as auxiliary / modal verbs in English.) I think what's going on here is that there is another verb wot, same meaning, which though dervived from wit is not a preterit-present verb, and therefore declined as normal verbs are in English. It's all pretty complicated, and wot tends to be dialectal in the north of England and Scotland.

Witting not, unwittingly yes.

A bit more on the etymology:

akin to Old High German wizzan to know

Now there is nothing much that's new in this Old High German connection, but it bears mentioning that the 'wizzen' does put a new sping on 'wizz-kid'--!