I think you're right about Slither---Hiss!, but it would be nice to keep it to level 1. Illusion [figment, fear]. It's a cross between ghost sounds and magic mouth, so level 1 makes some sense there. Suppose we tone down the fear effect to bane, lasting 1 minute. Would it still be 1st level in that case?
Pretty---Boom! should definitely be evocation [force]. The question is what effect exactly?
Gosh, I seem to have missed a lot of drake action!
I fear it would be difficult justifying slither-hiss! as a 1st level spell, since it is a multi-target fear effect it could end up a lot more potent than the 1st level cause fear spell. I guess if it only causes a few rounds of the Shaken condition and has some HD limit (affects total HD up to caster HD? (max 10)?) it could be balanced for 1st level.
Pretty-boom! looks more like a 2nd level spell to me, a downgraded version of explosive runes.