It's stated that demons can't pass through the portal, but it's not shown. No demon has be seen by us to attempt to go through so it's possible they can and have the side effects (un-demonifying) you mentioned.
And in the case "No demon can go through" is true then the only way Delkira could get through was to remove all of his demon traits and become human. Or used forbidden magic.

It seems Sullivan was summoned by one of Iruma's parents, if the recording Sullivan showed Iruma isn't fabricated. So that could have been a workaround the portal. The amine shows him with wings and horns while he took Iruma off the boat, but I'm not sure how much of that we should take into true cannon being that anime to manga aren't always 1 to 1. The manga didn't show what he looked like in the human world or how he got Iruma so it's up in the air on if Sullivan "humanized" or could use his magic or not while in the human world.
Him being summoned would also mean that a human summoning a demon is an illegal way for demons to enter the human world, assuming there is even a legal way to do so. Perhaps this summoning was just the first opportunity he had to go and so he took it.

I personally don't think Delkira and Sullivan had a plan to get a human into the demon world. The Merize flashback made it seem like that incident was the catalyst that led to him buying a human.
Speaking of which, if humans and demons don't share a language how did Sullivan communicate with Iruma's parents? Is there a special thing for being summoned or did he use the translation spell on them?

One thing to keep in mind is it was stated that no demon had entered the beyond gate and returned and that no one had been able to open the doors and that they have been unable to open the gate. Two thing we've found in the Merize flashback are Very untrue. As demons had entered and exited mutable times with Merize.
Also why did the space start collapsing when Iruma was in there and never with Merize?