What do I use to get the same nice seal and be able to remove the new seal when i have the car painted?

Buy a new seal, try to get a quality OEM replacement, some of the cheap ones are bad and fall apart at the seams. Use 3M black weatherstrip adhesive to attach JUST the corners of the seal, just a dab at each corner is usually enough. This will allow you to yank the seal when you need to without damaging it if you are careful.

Dont glue it in after you have the car done. You want to be able to pull it out each time you wash it, so you can dry the channel.

Mine won't stay unless i glue the corners in. I had to glue them one at a time and stretch the seal into place for the next.

Is the concern here that the water will collect in the corners by the headlight buckets? When I get MY car painted I plan to cut a notch in the channel by the corner so the water will drain in behind the headlight and out the drain hole. Same for the bottom channel between the headlights, notch the corners so it will drain behind the bumper. Bad idea?