You use the line out at the loadbox and go into the Kemper return. That type of profile would be a DI profile and then you would add a cab or IR that you like. The connection detail is shown in the manual but that is what you would do. Guitar into Kemper>>Send on back of Kemper to Amp>> Line out back to Kemper. Profile, refine, save, play.
and the sound interface? I would use the torpedo plugin WOS for the cabs..