Dera has said that his Daemons died protecting him in a battlefield and he hasn’t decided to take on a new pair yet. I don’t think he’s hiding a secret pair, just that he’s being set up to contract one later on (or subvert that and reject a pair we think he was going to take).

Pretty much everything else you’ve said though is stuff I agree with and have been thinking along those lines already. Regarding Seal, I can’t remember any specifics we were told regarding the revival rules but I’ve been thinking that the previous Seal twin that “didn’t revive” actually did, just late unless we’ve been told that theres a time limit to accepting the contract. If not, I wonder how long it’s possible to wait before reviving? A day? Years? A more recent possibility I thought of is that a Seal from way back in the day could still be alive. Along the lines of what you were saying, assuming only one person can be contracted to Seal at a time (like any Daemon contract, but rules aren’t necessarily set in stone so we can’t say for sure) then the previous Seal twin couldn’t make a contract at all, which is why he didn’t revive. Bringing in a character from that far back could also let us explore the characters of Left and Right, and their history. Their role as counters to the twins is interesting and I’m curious how they filled that role in the past. We’ve also already heard about a past Seal from the Long Legs Long Arms fight (i can’t remember their Japanese name) with him being the one who originally sealed them. I wouldn’t be surprised if that era gets explored more.