1. One. Don't see a need to split mileage.

2. Skipping the security lineup. Gives me an extra hour at least to sleep in every time I fly. Second? Well, the OP only asked for the first Upgrades are nice but a seat is a seat and early boarding? Well, that might save you from having to check a bag 10% of the time, but the lineup to get on a plane doesn't require you to get to the airport early. Dunno who that would be the biggest benefit.

3. No, and wouldn't really want it. If you don't fly enough to get elite status, then who cares about elite status.

4. Yup. I'm a domestic flier and well informed enough to ditch UA. CO hooked me up within two days.