"Whom among them...?" Sandy is right - and has given the best, simplest, explanation I've seen. I'll remember that.
On the grammatical level: 'whom' is English accusative (objective) case of 'who.' We don't use it with the verb 'to be' because that doesn't take an object; it takes a complement. Prepositions (by, with, from, to, for, etc) also take an objective case.
On the linguistic level: we never eliminated all the inflections from Old English, because they have their uses. People who call our grammar 'contrived' are perhaps posturing because they've learnt that word, and they may not know all its meanings. 'Her' would presumably have 'we' demanding things like "Who book dat?" Or "Fe who it done do this over where?" There's nothing like applying deconstruction to someone else's language, don't you know?
btw: I say hold onto the lovely old Northern pronouns! Foreigners just don't understand how really formal they're being by insisting on 'you' (the plural) instead of 'thee' and 'thou' (the singular)