get a piece of thin plywood or similar about 8 feet long, put a brick under the middle and one on each end, then unroll the tube across the wood. It won't get it perfect but the reverse curve should straighten it pretty well. If it's still bent then add more height in the middle, if it's bent too far then take some away. If not long enough then use a slightly thicker piece that is longer and more height in the middle. you can also help keep it straight by nailing some 1/4" thick strips either side of where the tube goes, or on a thicker piece run a router with a 1/4" bit from one end to the other. The gap must be wider than your tube or it will bend when you pull it out of the gap.
Yep, I unroll it unto a piece of angle iron and then use a piece of dowel that will hit the sides of the angle but not crush the tubing then just press down.
Wristpin, what you are describing is what I'm using throughout the car. It forms easy and I like the look. The only downside is that if you have to flare it every flaring tool that I've seen mars the finish leaving shiny metal showing through which I ***ume will rust. That's why I try to layout the tubing so I don't have to cut and reflare.
I never knew it was that hard to do a neat job with the brake lines. A sharpie and hand bender clamped in the vice I can lay them in there like factory. Just take your time , put a dot where the bend needs to be ( rotation ) and go to it.