plastic welding is easy, so easy in fact that i can do it, lol, you just need a good soldering iron and a steady hand (which i dont have) most bodyshops should be able to do it, its not a complicated skill
arch rolling is when the lip on the arch is heated up and pushed in out the way of the wheel, this is best when your car is lowered with after market wheels to stop the tyres from rubbing, its a rubber roller on an adjustable arm that bolts to your hub
yes, the valances can be plastic welded, this will make them more solid, however, they are seperate for a reason, if you clatter your valance on a curb, you can unbolt the valance, if you clatter it whilst its plastic welded/smoothed, you will need to change the bumper