I use a spray bottle with vodka & distilled water when I'm pressing and it works well for me.

My wool pressing mat does a great job on the creases for small pieces if fabric.

spray with water or starch...

I have found that Faultless Sizing spray starch will remove the fold.

Vinegar works. I wouldn't worry about residue. There wouldn't be enough of it to do anything.

When I take out a piece of fabric to see if I want to use it or if it enough or for whatever reason, I always fold it a different way when I put it back. Try to get the folds in a different place.

I use Sta-Flo. I dilute it with water: 4 oz. Stay-Flo to 12 oz. water. I use the bottle (from the expensive spray starch) that I've used for at least 5 years, and mark the oz. with marker so it is easy to see. That is a very good sprayer. Diluted that way it costs me .04 cents an ounce.

I was going to say the same thing as "IceBlossum." Sometimes what we think is a crease is really wear and tear and/or sun damage. This cannot be removed. You have tried any remedies that should have allowed you to press out a fold line. Save that fabric for smaller pieces that don't include the fold area.

I use Best Press. More expensive than vinegar, but does the job perfectly.

I spray them with water and press ..if they don't fully come out, I still use them because the first time the quilt gets washed it will come out anyway. Don't stress too much over them, just press them enough so you can cut accurately and you'll be fine.