Learn English – is “through whom” grammatically correct

From an American English perspective:

C1 and C2. You have a glasses fetish. :-)
C3. There's something about girls who wear glasses which you like.

All three of the B's are idiomatic.

A1 and A2 are to my ear unidiomatic. They sound like Indian English to me.

A3 is fine.

Have is already doubly continuative

1. they possess (meaning-wise)

2. Their general ongoing state of affairs is to have excess time (tense-wise)

   so continous "having" sounds pleonastic, except when you are having fun, or are having a hard time of it, in which case "having" means "experiencing" with the implication of "for a temporary duration", whereas "possessing-having" seems permanent.

The first is nearly correct, but it is conventional to put the preposition with before whom:

Do you know any people with whom I could combine my efforts?

Here is a question from EL&U that justifies the use of whom, and incidentally shows where to put the preposition.

Your second option is correct but somewhat unsatisfactory: it is just a way of avoiding the issue of whom.

The third is also correct and equally unsatisfactory: it is the same as the second apart from exercising the option to drop the relative pronoun that. This is just a personal opinion, but I think that it's OK to exercise this option in spoken English but it's a bit slack for formal, written English.

Note that there is no option to drop the reltive pronoun who or whom.