If done properly,.. silicon or any other adhesive won't be needed. I've done 2 pairs of diy's, and I've done a TSX retro ( I had to open my headlights up about 10 times), and I havent had a problem. It's been raining in NY for 8 days straight, and niether lamp has fogged up.

As far as the bumper coming off,...
after you you paint your lights, leave the bumper screws off so you won't have to go thru that again.