My current thoughts are that this John is our John, and that Rodney and Teyla have come to rescue him. Sumner is Goa'uld, and the marines he brought along either Jaffa or young Goa'uld.

I think Zelenka doesn't have glasses because he's carrying a symbiote too.

One of the things playing at the back of my mind is when Kalowski got overtaken with an immature Goa'uld symbiote, and how it could only take over the guy occasionally, strengthening as it got more mature. If John has an immature symbiote in him, it would explain the white outs (triggered by the mention of tretonin) and the struggle he's having with whatever the infection is (or symbiote or whatever) he's got.

So, spinning a bunch of things here, someone is attempting to clone symbiotes, like the ones used for creating tretonin, as they have no queen. They plan on then taking over Atlantis, which is why they really want John's knowledge, and have given him some of the immature, experimental symbiotes in order to gain access to that information. But the creatures aren't strong enough to completely take over our John.

Enter, you know, the team and the rescue attempt. If could still be a sequel to refraction, only their own John is dead.

Just, you know, speculating here.