Conjecture Question..Any evidence to support/reject this theory concerning Echo?

[Yrusama]

Wouldn't leaving a strong enough impression change the past then? at least for that person?

There's a big difference between planting a memory of your presence and actually changing an event. However, I feel like the THM36 quest had a situation where the errant Lalafell THM was made aware of your echo jumping, and was able to anticipate when you'd show up. I'd have to watch those cutscenes again, though.

In any case, it's as simple as the difference between planting a memory of your presence within the present-time Echoed person and planting awareness of your presence in the Echoed person's past self