Yes, I am going to transplant it or dig up a rhizome soon, so I can plant it in the garden in full sun. I think it has been in part sun most of its life, but other Yucca flaccida plants right near this one (in the same growing conditions) are much broader in leaf form.
So I think this narrow form is likely a genetic variation of Yucca flaccida.

Yes, it would be good for breeding more narrow leaf Yucca plants.

The Yucca is probably 1.5m above the creek. In high flooding weather (maybe 1 time every year or two) it can reach up to the Yucca plants and that leaves behind sandy sediment from the creek water.
Also the bedrock is not to far down, so most soil there is largely made up of old sandbars that have accumulated over time.