The best way to manage it I have found is through intervals.icu indoor and outdoor ftp number settings. I have tested this numerous times and my assioma duos read 4% lower than my kickr core and so i adjust the 2 ftp’s and Intervals picks up and adjusts the IF (etc) as the rides come in.
I know it’s not perfect but after years of riding i am pretty good at gauging what the various zones feel like. To answer your 3rd point, i’m not sure there is a solution that i can think of. You definitely can’t test it on the kickr?