Syntactical ambiguity in introductory phrase reference: reference to main verb vs. object

"Outline" in this usage is a verb and it has nothing actually to do with "creating an outline". Your two example sentences completely change the meaning of the source sentence.

A motivated us to outline our proposal for B.

This doesn't mean quite the same thing as the source sentence but is close enough for your stated purpose. The subject ("us") is motivated to outline, not motivated to create an outline.

Motivated by A, we outline our proposal for B.

Does "Motivated" refer to outline or proposal?

Neither.

"Motivated by A," is an adverbial phrase - a free modifier - that modifies the whole of the main clause "we outline our proposal for B."

Thought & Co, the grammar website, has a good explanation. You should also look up "bound modifiers" to see the difference.