These are basically fixed constructions in English. You are friends with someone. Note the -s! Also, the with is not optional.
You are free to use whom or leave it out. Who is technically not correct in this case. Not everybody will consider it incorrect if you use who, though.
These sentences are all correct, read them carefully to understand how they are different and how they are similar:
These are people who are my friends.
These are people I am friends with.
These are people with whom I am friends.
People I am friends with, should be...
People with whom I am friends should be...
People whom I am friends with should be...
As for you other sentences, the verbs date and move in are not used in a similar way, so you cannot use them in a similar sentence! You date someone (not with someone), but you move in with someone (moving in someone would be possible, but it is a very graphical description that you might want to keep to yourself unless you are a baby moving inside your mother!)
She is not the kind of girl I would date.
She is not the kind of girl I would move in with.
Prepositions don't have to be part of a phrasal verb, in fact I would say most aren't. In this case, none of the prepositions have any special relation to the verb. They're all literal descriptors of the positions the subject goes through during the action.
Try to break up the sentence into the smallest chunks that make sense. Here: He fell. Yeah, that makes sense, that's true. Okay, where did he fall? Into the pool. What did he fall off of? He fell off the ledge. So now, put it together.
He fell off the ledge and into the pool.
You can leave out the "and" here, but it's still there for grammatical purposes. You can tell if you use three prepositions:
He fell off the ledge, through the air, and into the pool.
You can't leave out the "and" here, there's no way to use three prepositions without the conjunction. That means that there's no way to do it with two, either, and the "and" is still there, it's just not spoken.
Right and straight are both intensifiers, and don't really make sense to me here. There's not exactly an indirect way to fall off a ledge and into a pool.
I hope this helps.
Edit: A note to help distinguish between phrasal verbs and normal prepositional adverb phrases: phrasal verbs don't accept objects of the preposition, and they don't pair with prepositions in conjunction phrases.
The roof fell in. good
The roof fell in the house. not good
The roof fell in and onto the ground. not good
The roof fell in onto the ground. good
Does that help you see the difference a little better?
Best Answer
It depends.
I would say that the following seems more frequent for your example:
If you are adding something, you may say:
If you are telling that some track isn't there, you may say:
onto is generally used when there's a physical movement.