Point 4 - I would say that if you are checking in with AA as they are operating the first sector then it is more likely than not that AA will check you in and issue boarding passes for all subsequent BA operated sectors. These can be printed at home. The BA app will show you as checked in but will also give an error stating that it cannot confirm your check in status (!) and therefore cannot issue a boarding pass. The BA app will, in some situations, actually allow you to download the AA boarding pass for the first sector. In situations like this you generally can't use ba.com to check in for the BA sectors either, as ba.com will redirect you to aa.com to check in as they are operating the first flight. If you need a BA boarding pass for subsequent sectors you will need to get these at the airport. Even after BA have issued the BP manually the app will still refuse to display the electronic BP.