The angst here is more in the course of the story, not the characters. It's not knowing what's going on. As you say, their relationship is solid. Very much so.
There are some OOC-Justin moments here, but I looked at it a bit differently. We know Justin is strong, and is stronger at the end of S4. But we also know how he reacted to his "gimp hand" during S2. He was distant, angry, and indulging in some self-pity. How would Justin react to having been accidentally possibly infected? We really don't know. So I'm willing to suspend that disbelief, because if it were me, I'd be freaking the fuck out, and not telling anyone until I knew for sure.