The reset on the levels of the current enemys make sense, even though I really dont understand there desicion to make all the enemys just vanish at random. I would figure they would just respawn at a higher difficulty rather then just all dissapear to up the level at once. I'm probley just not noticing the level increase seeing as how a DC can be a couple levels difference.
The reason they gradually despawn (several times) and gradually revert back down to their lower/original levels is because people bitched about high level mobs being left behind at camps that they used to be able to farm weaker mobs at. It wasn't that way at Abyssea's release, it was something that was added in later. It's more of a hindrance than a help nowadays because way more people are at the level cap now and more people have the good atmas and such from Abyssea, but it's also not SO much of a hindrance that people are going to raise enough of a stink about it to try to get changed back to how it was before.
To me, it's felt like Dom Ops favor giving out seals that are dropped by NMs related to the mobs that you kill for the Dom Op. For instance, when I've done the Dom Op for Mechanical Menaces in Abyssea Uleguerand, I feel like I get a lot of the seals dropped by Koghatu. But I don't have any solid numbers to back that up.
Thing is, not all the ops have a local NM that is "related" to the bunch and drops seals. For instance, the raptors in Ule only have Kur hanging around them, which doesn't drop seals.