I think that nowadays "whom" is most often used when it immediately follows a preposition: to whom, for whom, etc. Many speakers will not use "who" in such cases, although they will in other cases where it is an object rather than a subject.
1. For whom is it intended?
2. Who is it intended for?
3. For who is it intended?
4. Whom is it intended for?
You do hear 3 and 4 from native speakers, but I think 1 and 2 are more common.
Of course 2 and 4 get us into the question of ending an English sentence with a preposition -- but that is a different can of worms.
You’ll get no dispute from me that “who” is more common. I try to use “whom” whenever it appears as an object: “Whom are you meeting there?” “Whom can we expect for New Year’s?” and so on. If that sounds pedantic, pretentious, or old-fashioned, well... so be it. At least I avoid “whence” and “whither”.
As for ending sentences with prepositions, I assume that most native speakers are on the same page. I do that profligately in the spoken language (and see, e.g. the Scandinavian languages: Hvor kommer du fra?). In formal written language, probably thanks to centuries of influence from Latin grammar crowding out the pre-existing Germanic, I try to avoid it like the plague: “For whom is that intended?”, rather than “Who(m) is that for”, for example. (And I admit that “Who is that for?” now sounds more natural, and I’m probably as likely to use it if I’m not thinking about grammar...)
In English, some verbs are not so often used in continuous tenses; see is one of them.
"Whom are you seeing?" would usually imply "Whom do you meet regularly, date, go out with?" rather than "Whom are you perceiving with your eyesight?"
Similarly, we say "I don't think so" rather than "I am not thinking so", and "I have a book" and not "I am having a book".
Right, and what complicates this is most verbs in the simple present form are used to denote something that just generally happens: "I eat breakfast around nine" "I walk in the evenings" where the continous is mostly reserved for the currently-happening "I'm walking, I'm eating." Ie in many usages we use the present for the continuous and the continuous for the present. this is one thing I see still tripping up otherwise fluent non native speakers.