The scenario I had only a few weeks ago, referee incapacitated after 20 minutes, other AR was bench side & senior, albeit we are both L5. Interestingly the stand in club AR was offered up by away team, so he monitored his ’own side’s’ back four which was bench side, I stayed put the other side. Second half the home side found a qualified referee, affiliated to their club so he stayed bench side in 2H. Referee needed treatment so could not stay as 4th official, so we decided referee would manage substitutions. After all that ended 0-0, 22+ handshakes!