I never fully liked the level of gimmickry in his defenses. But even if you love Manny as DC, I don't see how he can be both HC and DC.
No expert on how great DCs become great DCs. But it feels like it starts in the war room watching film all day long. To call a defense, don't you want to know every single player's tendencies from the opposing offense by watching every single play they've had over the past several games. That's 11 players x ~70 plays a game x ~3-4 games. = 2-3K snaps.
Then the DC should look at his own players and know their tendencies to identify how opposing OCs will try to exploit them. snaps to watch. Probably a little less snap counts to watch, since it's the same players every week. But still need to put in the hours.
That's a lot of time in the film room. Then you add in managing the staff and recruiting and coaching and developing the players. 70-80 hour work weeks in-season should be the norm. How the heck do you manage that and be HC? Impossible IMHO.