^^^ You should be more worried about Tachy Cardia with 200mg+ of fucking Benadryl. Jesus man, come on.
First off, I don't know where you got those figures from, diph and seroqeul have roughly the same affinity for H1 receptors. Secondly, the delirium from diphenhydramine is not caused by H1 antagonism, it is the result if it's anti-cholinergic activity.
WD symptoms from 600mg of seroquel(Q), cannot be treated with megadoses of a 1st gen H1 antagonist. I repeat, ANTIHISTAMINES ARE NOT A SUBSTITUTE FOR LARGE DOSES OF SEROQUEL.
Seroquel is an antipsychotic, despite how ineffectual it may be in small doses used for sleep, at larger doses(200-800mg) it is indeed a quite competent dopamine/ serotonin antagonist.
Diph isn't going to cut it in that regard. I'd strongly advise taking a few days to taper the doses. Also going even 72 hours without any CNS active H1 antagonist's will cause your tolerance to the hypnotic effect to drop incredibly fast. After that, I would try 12.5-25mg doxylamine to help with sleep.(absolute max 50mg).
And not to be rude, but at 16, you're hardly qualified to make these kinds of adjustments without assistance.