1 - Have a good read
here.
2 - It's do-able yourself. You don't need to remove the whole exhaust, just the front components (OP/FP) to make it easier. The fitment is designed to be a direct replacement of the OEM manifold so yes, any exhaust system is fine.
3 - Gains for UEL / EL are likely to be similar, it's down to boxer rumble (UEL) or not.
4 - No idea, I have no idea what USA emission laws are
