Hehe, always happy to answer AU questions!! And the vampire AU holds a special place in my heart for being my first Murdle AU ^^
1) With garlic, it's probably a similar deal as silver- If he eats a little, he'll get sick, but overall recover. A lot would make him sicker. Also, with his heightened senses, its smell alone is really offputting and overpowering, so another reason to avoid it. With lavender, probably not? Just because I was re-reading the Vampire Logico fic to get my lore and details in check and Irratino has lavender soaps/perfumes (I've heard of the garlic myths, but not as much the lavender ones!) and it'd be a bit of an awkward retcon to make him uncomfortable with lavender haha.
2) He does indeed! Vampire Logico is notably faster, stronger, and has keener senses than a human. He doesn't need sleep, and he heals much quicker too- Plus, those injuries don't scar at all. The last scar he's gotten is the fang mark on his neck, matter of fact. (He'll write this all off as just "an evolutionary advantage" and "vampires are pursuit hunters instead of persistence" and whatnot.)
Vampire Logico is also immortal! Though, he's got a few more years before he realizes he doesn't age while everyone around him is- He's frozen in time. He's not going to die unless someone hits him with vampire hunting gear- Silver bullets, fire, stake, the like. (though, as Logico is keen to point out, that will kill anyone regardless of vampire status).
Sadly, he doesn't have the usual vampire powers like telepathy, shapeshifting into a bat, enthralling anyone, the like. Irratino's maybe secretly a little disappointed. Not that Logico needs any supernatural charm to have him hooked.
Thanks so much for the ask!! ^^