[@Lucius Cypher]
The summoner part is my main problem. I mean I'm pretty sure Avery is the only one who 'summons' in the entire Re Zero AU. Sure there might be other spiritual art users, and thus they can bring forth a spirit. But they're usually restricted to just one (hence why Avery also only has one contract). Avery in theory can make an unlimited amount (of course how many he can make in a day is very much limited, just talking about over the course of his life depending on how long he lives)
That was something I couldn't bother bringing up, as that was the least worrying one to me. But I agree, how the heck did he get in?
I mean the Celestia's are elitist douchebags. They're not going to just leave the front door open for anyone and their grandpa to walk on in, there would be magical traps, runes, guards, guard dogs, [s]possibly a dragon or two[/s]. You know the usual fantasy security systems.

Also to be fair to his skillset, you forgot to mention at least a good memory to remember information.