Learn English – Set Off For/From

First, "of my heart" simply means that something is associated with your emotions:

You are the love of my heart.

Also, when we use words like "heart" or "head" metaphorically, prepositions may not mean what you think they mean. Consider:

You really got in my head for a minute there.

That simply means, "You affected my thoughts," and (thankfully) has nothing to do with you shrinking and climbing into my skull.

As for "of my heart" vs. "in my heart", here's one way I might characterize the two:

• of my heart can be used with the definite article, to indicate preeminent personal feelings
• in my heart can be used with the indefinite article, to project a role

For example:

You are a diva in my heart. (means: in my heart, I regard you as a diva)
You are the diva of my heart. (means: in my heart, I regard you as the diva)

So, back to what you said:

"of my heart" sounds like you have several hearts, and one of them is your champion...

Instead:

the champion of my heart

means something more like:

there may be several people who could be my champion, but, in my heart, you are my true champion

Disclaimer: I wouldn't regard this as a hard-and-fast rule with zero exceptions. As was mentioned, prepositions are very tricky. Macmillan lists more than 20 possible usages of the word of, and when you combine those with metaphorical words such as head, heart or skin, some preposition-noun combinations will indeed be idiomatic, and need to be learned one-by-one (such as, "She really gets under my skin.")

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?