syntactic analysis - Traditional explanation for 'I don't remember whom is whom'

I'm looking for a traditional grammar explanation in regards to the sentence: 'I don't remember whom is whom.' Also I'll preface that I'm native speaker for whatever that's worth...

Now I understand that the most natural way to say this is, 'I do not remember who is who'. From that, because it's the accusative case, one should normally say:

'I do not remember him' -> 'I do not remember whom'

However, since 'is' is a copula or linking verb, the following explanation by Grammar Girl applies. My understanding is that it forces the same grammatical cases, so 'whom is whom' and thus:

'I do not remember whom is whom.'

In another 'who vs whom' question, one person suggests 'Who is who' is correct because it is a set phrase. Though I suppose I can accept that set phrases exist as a thing to torment learners (set phrase as it more means 'which person is which' or other things apparently), at the very least the why it isn't 'who is whom' can be explained by the Grammar Girl link earlier.

Also a side question: ignoring who vs whom, would it be more correct to say, 'I do not remember who being who', so leaving the verb in the infinitive form. As an example, 'I remeber him being another person.' Or perhaps the infinitive 'to be' is the most correct? At this point I don't trust my English enough to tell, and I might probably just analyzing this all wrong anyway...