I have not seen any actual splice protocol documentation either (if there is, can anyone link to it?).
In particular, we do need some kind of splice_cancelled which allows backing out of a splice where the splice transaction is definitely not going to confirm anymore. If our only possible reaction to a splice transaction definitely not getting confirmed is to close unilaterally, then a batched splice is still risky (whether or not you are batching with 0-conf / JIT channel opens) because if I batch-splice with N participants, any one of them can cause all the N-1 other participant channels to unilaterally close.