Watson, I know nothing about longarm quilting but your top is amazing! Please show us when you finish.

Not a long armer, but WOW! What an amazing quilt.

I think your plan is a good one.
As for the white feathers - they will have texture with the white thread, if you're looking for a sharper contrast, then you might consider the soft gray thread that someone else suggested.
Personally, I would do the white because you don't want something dramatic that will draw attention from the stars??? Or maybe you do? Your choice!

I think the concern with stitching in the ditch is that it will add lines of texture that will interfere with the beauty of the feathers.

I can't really help with the LA'ing. Your quilt is beautiful and I wouldn't mind having one like it.

I have a longarm. When I was learning I asked the sales person the best way to handle your question and she said she always worked everything across a quilt and then rolled to next section. I've always done it this way and don't have problems with wrinkles from rolling back and forth.
As someone said it's a toss up between changing thread colors or rolling and re-rolling. I go with the thread changes making them as painless as possible(last color worked on last row is first color used on next row, etc). I find the rolling forward and the steps involved more daunting than thread changes.

I love your quilt and the humility block is just perfect. Lots of people do the squares and then quilt other details in. I don't really care for that. Seems to make it too busy. I'd just go with the feathers if that is what you want to do. Having said that - doing an echo in the stars themselves and doing feathers in the rest of it could be very attractive..