answer to question 1 : In formal English the preposition is placed before the relative pronoun "whom"- ...the people with whom I went to school.
answer to question 2: In spoken English we normally use who or that instead of "whom". In this case, we place the preposition at the end of the clause - "...the people who I went to school with."
answer to question 3: the relative pronoun (whom) can be omitted when it is not the subject of the relative clause which it introduces: i.e. the subject of the verb "went" = I, therefore "whom" can be omitted
answer to 4: the two sentences can be combined if they have either a common subject or a common object. Try it and see what happens.
answer to 5: see n.1
I found an article, which tells us to use 'who' placing the preposition at the end and 'whom' preceded by the preposition. However, it is mentioned that the usage of 'whom' is getting obsolete.
I really appreciate your help!
People have been predicting the demise of whom for 100 years; yet it lives.
I prefer "whom" because it is correct; "who" is OK in informal use. Your best bet, in that sentence, is to leave both of them out. "people I went to school with" or curmudgeon's "that I went to school with".