I think the 1280 no longer supports on board logging? Need the 2560.
You can still log the telemetry on the mission planner. For most purposes this is enough
Yes. No logging on 1280s--the APM firmware footprint is too large.
If you need specific logging, then you can modify the code to get some of it while commenting out lines that are not used (e.g. sonar/optiflow/some auto mode stuff)... It's possible, but you'll end up with custom firmware which could break on every new release.
Best way to get some sort of logging is get wireless telemetry and record via tlogs on mission planner.