That's a very sweet picture! If he vomited dry food, I think your idea of taking most of it away is a good one... he is eating wet food, right? If only from the syringe and your hand? (I think some cats like being hand-fed when they're not feeling well: our previous cat ate off cutlery and from my hand when she was sick. She wouldn't even lick a spoon when she was healthy.)I guess I'll pass updates as needed.
We've had a vet tell us that purring can be a sign of all sorts of emotions.
Over all, it sounds like there are lots of good signs for you--in his eating and apparent comfort--but I would also be wondering about the questions you raise regarding the blood test and toxoplasmosis. I hope the vet can help you by answering that and suggesting what to do next.
Thinking of you!