If you are portraying the situation accurately, then I would say that the rescue you are dealing with leaves much to be desired in the realm of sensitivity and concern for the dogs they place. Having said that, you also could have taken him to your vet and had the uti confirmed and treated...since you are in the medical field you must know that urinary/renal issues can profoundly affect and exacerbate mental health in humans, and therefore possibly dogs too...on top of a seizure disorder this should definitely have been worked up. I know you likely felt overwhelmed, but you literally gave this dog no time to acclimate at all. I hope the rescue listens to your description of his behavior and does right by this boy by having his medical issues properly worked up. Don't know what you can do about how you're feeling about what happened and how you handled the situation...except possibly learn something from it. Any dog, if not aggressive or dangerous, deserves at a MINIMUM, two weeks to a month of time to adjust, and a thorough veterinary work up to assess physical problems, if there appear to be any. My heart breaks for these dogs.
you said it much better
a min of 2 - 4 weeks to see their true personality come out
and if he was miserable with an infection in bladder or kidneys
that could have also made him have a flat persona or demeanor