Conflicting who/whom usage rules in a sentence [duplicate]

"Please gather information on who can serve as a proctor." This sentence is grammatically correct. WHOM cannot be used here, for very good reasons, which could be properly explained by the eminent grammarians.

As I understand it, it's quite simple: I (who am not a grammarian) correlate 'who' with subject and 'whom' with object, and originally though that it's a rough and incomplete comparison.

Examples:

WHO offended him? (subject)

WHOM has he offended? (object)

We know WHO played this trick. (subject)

On WHOM was this trick played? (object)

However I found the idea that 'who = subject' and 'whom = object' is well established: according to Grammar Girl,

When you’re trying to figure out whether to use who or whom, it helps to know the difference between subjects and objects because you use who when you’re referring to the subject of a clause and whom when you’re referring to the object of a clause. In other words, who is a subject pronoun and whom is an object pronoun.

Source: http://www.quickanddirtytips.com/education/grammar/who-versus-whom

Supporting quote:

“Who” and “whoever” are subjective pronouns; “whom” and “whomever” are in the objective case. That simply means that “who” (and the same for “whoever”) is always subject to a verb, and that “whom” (and the same for “whomever”) is always working as an object in a sentence.

Source: http://web.ku.edu/~edit/whom.html

In your question, the first part of the sentence, 'please collect information on', will not influence the choice of whether who or whom should be used within the second part of the sentence, 'who can be a proctor': in fact who is to be used in this case rather than whom because the word chosen needs to denote the subject. Here 'who' = subject, '(can) be' = verb and 'a proctor' = object.

However your sentence can use 'whom' if it could be rewritten with some modifications:

Please collect information on whom to consider for the post of proctor.

Here 'whom' is used because the pronoun chosen now needs to denote the object of the verb '(to) consider.'

Other examples (my own):

He hates his friend, by WHOM he was duped.

With WHOM did she study for the exam?

This year's semi-finalists are Britain and USA, between WHOM the 1999 Final was played.

For WHOM did the bell toll?

I am very surprised to see this level of confusion between who and whom: applying a 'substitution rule' and so forth! Maybe we should keep it simple, without avoiding 'whom' just on general principles, because the humble 'whom' has its own place in the scheme of things - if we never used 'whom', then "For WHO the bell tolls" would sound like a question put by a learned Indian (for who the bell tolls? The bell tolls for who? For who the bell is tolling?!)

A few more examples...

Prepare a list of those employees WHO can ride a bike.

Brief the students WHOM the Minister will be meeting tomorrow.

WHOM did the Financial Crisis affect? WHO knows for sure!

Find out WHO did it, and for WHOM; WHO will suffer as a consequence, and WHOM will it benefit?

I sincerely believe that 'who' cannot replace 'whom' in most of the above random examples. So we must READ A LOT, practise our usage, and find for ourselves the simplest ways to "keep straight" our who's and whom's!

Who is the correct word in this instance.

As sumelic mentioned in a comment, the word in question is an interrogative pronoun. This pronoun functions as the subject of the noun clause (who can serve as a proctor), which in turn is the object of the preposition on. The function of the clause, since it is not a participial or infinitive clause, has no effect on the case of its subject; hence, the proper word is the one that would normally be used as a subject. This, of course, is who.