I just tested this out in the builder to see for myself what the results are. These are the results I got:
*Normal attack bonus: +12 with a +2 dagger as an implement.
* With Serpentine Knife of Zannad (as holy symbol): +13, including +1 to attack from Holy Symbol Expertise
* With Accurate Serpentine Knife of Zannad (as holy symbol): +13, including +1 to attack from accurate dagger.
So it looks like, and this may or may not be a bug, that when you have the accurate dagger it overrides the "this is a holy symbol" clause and treats it like a dagger. If you don't have the accurate dagger the magic weapon is treated like a holy symbol.
I'd ask your DM, show him some of these replies, and see what he says. My opinion is that the combo should work. You're spending several feats for what is essentially a +1 to attack; I don't think that'll be an issue.
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