I haven't finished the book yet so I don't know if this is explained later, but...
>飛び去ったはずのワイバーンが忽然と姿を消してしまったのは、彼女が気絶したことによって召喚術の効力が消えてしまったせいだろう。
This doesn't sound right. If the summoner has to remain conscious for their summon(s) to stick around, how do はぐれ召喚獣 exist? How do 護衛獣 remain overnight while their master is sleeping? That counts as being unconscious, right?

Summon Night 1 and 2 explained a bit about はぐれ召喚獣, how only the summoner could void the contract and send the 召喚獣 back to their world, and if anyone else tried it could fail and send the 召喚獣 into the void or erase them from existence or something. The details of the contracts are never elaborated on aside from 1) requiring absolute obedience to the summoner and 2) weakening the 召喚獣 from their original state, but I had a theory that the unspoken contracts included conditions for return. For example, the contract for a 護衛獣 might say "protect me indefinitely until I send you home" whereas a normal attack 召喚獣's contract might say "attack this guy, then you can go home". はぐれ召喚獣 would be 召喚獣 who were summoned irresponsibly with no return condition and then the summoner died, leaving the contract valid but with no one to void it.

Anyways, on the subject of return conditions, I have a hard time believing ミニス's summon contract would say "protect me (unless I pass out lmao)". Or is the wyvern bound to any contract at all? If it's not bound by anything, that would be even less reason for it to vanish after ミニス passes out, right? And now that I think about it, there's a moment in Summon Night 2 where you void バルレル's contract so he can get his full power back and he doesn't get sent back to サプレス, so... hmm.........