The easiest answer is that the shootout at the end of The Devil's Rejects was some kind of dream sequence and they actually got captured. That's a totally lame crock of shit but it's the easiest way to bring them all back. I mean Zombie started his Halloween sequel with a fifteen-minute dream sequence, didn't he? I was just thinking about TCM 3 the other day and how it was so different from the first two TCM's (different family, frightened and beaten dog, mentally ill Leatherface is now suddenly idiot bully Leatherface) that they might as well have turned him supernatural to explain how he came back from having his guts chainsawed out in TCM 2. Also it would've at least given that awesome Excalibur trailer some relevance to the film if Leatherface was actually resurrected in the movie itself.
I do like the idea of a shared Rob Zombie universe if it means Richard Brake is playing Doom-Head in this. I figured he was just playing some demented cousin or one of Mama Firefly's many bastards. The Dr. Satan scenes in Corpses were pretty far out there. Dr. Satan and the mutant with the flamethrower certainly didn't look like remotely normal human beings to me. That whole underground lair scene is pretty disparate from the rest of House of 1,000 Corpses and especially from anything in The Devil's Rejects. To Technico's overall point, the Firefly clan never interacted with any of those mutant weirdoes so it was easier to just shrug off as random crazy shit that happened at the end of Corpses without affecting the main story of the Fireflies.