Learn English – “I’m waiting for you…” or “I’m waiting you…”

These sentences are grammatically incorrect as they're in the wrong tense. I am waiting for you is in the present tense. If you want to reference a length of time you've been in the 'waiting' state, you want to modify the sentence to use present perfect tense, as the length of time you've been waiting has already occurred. Correcting your sample sentences:

I have been waiting for you for so long.

I have been waiting for you for a long time.

In this context, once is acting as a conjunction, which is used to connect two clauses to make a sentence. The two clauses are

I will confirm
I’m at the office.

The two clauses are grammatically correct, and once adds the meaing that the first clause will take place as soon as the second clause occurs.

According to the Cambridge Dictionary, once can be an adverb or a conjunction. The adverbal usage is more common and has a different meaning: maybe that's what is making you uncomfortable about the sentence. You could replace once with the conjunction when, and the meaning would be almost the same, but without the sense of urgency.

I will confirm when I’m at the office.