I think the UA standby has to do with it being on AC metal. UA on UA metal allows you to get on a standby list while doing on line check in and if you want it confirmed you can do a confirmed standby for $75 3 hours prior to the flight via phone, and if there is room you are on.

If you do book via UA on AC metal, you very likely will not be able to mobile check or on line check and will have to do so at the airport.

I seem to always have problems with on line check in with one segment on the other carrier, regardless of who the ticket is through.

If it is all on AC metal I would just book through AC, and vice versa.