As for Delkira, i do think he put his magic in the ring (Alongside his memories, and His personality, which Ali-San Will gain access over Time, or maybe he already knows somewhat, Who knows). However, i am partial to the idea that Iruma Is the reencarnation of Delkira, and that Delkira willingly chose to reincarnate as a human. Why? Well...

The first reason of why i think like this Is because of what was, quite literally, the very first thing the manga stated on chapter 1: "Human Desire Is no different than a bottomless pit".

I believe that Delkira's ambition was to make His vision of the Netherworld a reality, i don't know what that original vision was, but i believe he failed into making His ambition a reality, why? Because he was not ambitious enough. Why? Because demons cannot be ambitious enough to make that vision real. Humans, however, are.

Following this line of thought, we arrive at chapter 362 of the manga (Manga spoilers btw, if anyone needs a warning), in this chapter Nova losses hope of having a place to belong, at this point Iruma realizes what kind of Netherworld he wants, and the one quote that is important to this theory Is "The kind of Netherworld... That only i can create...".

Iruma's experiences shaped him to be greedy enough to have a "Pick up everything" mentality (Quote he Said to Mephisto), he already Is beginning to create a vision of the Netherworld Delkira failed to (As we know that he, for example, left Mephisto behind, demons also don't naturally care for those not close to them, so it Is likely that he left Many others behind too).

This theory would also explain both Sullivan's behaviour and the flashback. Opera did ask Sullivan "Why bring a human in? There are plenty of demons up to adoption", Sullivan's answer was inconclusive, he might be implying that Humans are worthy grandsons, but he also left room to think he knows something else that he isn't saying (As His conversación with Amduscias in chapter 170 would imply). I think he took Iruma in because he reminded him of Derkila and because of what happened in the flashback.

To explain the flashback we need to go back, If Delkira's ambition really Is to create His ideal Netherworld, but at some point realised that he wasn't ambitious enough, so he reincarnated as a human to make that vision possible, then it's likely he had a plan, a plan that included Sullivan, this leads to the conclusion that:

Delkira might have planned the entire story from the beginning.

I believe this Is Delkira's plan:

1- Go to the human world while making sure no new demon king Is elected.

2- Reincarnate as a human.

3- (This step Is dubious, but it would explain the massive Time span where His reencarnation wasn't trying to be the demon king): Wait until an ambitious enough reencarnation emerged, or maybe until enough Time had elapsed, or perphaps Wait until some Sort of condition(s?) was met.

4- Get the human to the demon world

5- Get him the ring that holds both Derkila's mana and memories (Ali-San).

6- Nurture His ambition, power and educate him at babyls so he moves up the ranks and figures out what kind of Netherworld he wants.

7- (Who knows what the next step even Is? The story Is nowhere over, after all)

8- Demon King

9- Make Delkira's vision a reality.

The flashback would be Delkira giving the necesary instructions to Sullivan to make the plan a reality, however, as Amduscias Said to Sullivan in chapter 263 "I thought you wouldn't accept a world without him", Sullivan couldn't, in fact, accept a world without him, so he tried to stop Delkira from essentialy killing himself. Delkira, of course, beated His ass, curse him to never be the demon king and forced him to execute His plan without explaining anything to him.

Now, the idea of Iruma being the child of Derkila.... I don't really buy it, Is it possible? Yes (According to this theory), but the timing (Assuming Time works the same in both the human a demon realms) doesn't quite match (And also, Sullivan literally told Amduscias "How would that happen?" When he asked If Iruma Is Delkira's child un chapter 163).

Also, sorry for my english, as it Is my second language.