No, not diabetic. So my skill with injections is limited.
You are welcome to narrate Jekyll's role in the invasion as much or as little as you like Scott. He's there, he simply is not taking part in the exact same battle the rest of the PCs are and the ultimate outcome will depend on their actions. In other words, Jekyll can take part figuratively, but he cannot influence the outcome in a direct way, like saving a particular NPC that the main party fails to save, or stopping a disaster the main party fails to stop. Now, he could encounter them briefly between encounters, much like Sheriff Hemlock has just encountered them. Perhaps Sheriff Hemlock even saw Jekyll fighting goblins in the crowd (if that is how you want to narrate it) and deputizes him as well but sends him elsewhere, perhaps taking him to the East Gate. And in a grand melee, he might even (narratively) engage with the main party momentarily in some small way that has a negligible effect on things. To summarize, he can be involved in the narrative, but not the mechanics.
This actually brings up something I wanted to make known to the rest of the group, although it is only tangentially related. Even though this is an Adventure Path, I do not tend to run games that are tightly structured around a single story. I am mixing the "Adventure Path" concept with the "Sandbox" concept a bit, such that Varisia is your sandbox. Although there will be a main story which is the adventure path, the world of Golarion is alive with other stories to be told, and you will be allowed to pursue those stories if you like, although they are likely to be relatively short. I am very much a "journey over destination" person. My goal is not to finish the adventure path, but to enjoy telling a great story with my players. If the players choose to pursue the main story over the others, there's nothing wrong with that, but neither is there anything wrong with deviating from the main plot if you enjoy it.