1. Killdry (as the name implies) just deletes the dry signal. So, if you have 10% wet/dry, you will get 10% of the maximum wet level. Killdry is mainly intended for use with parallel effects loops, where this behavior is appropriate. It also applies to a send/return from a mixer.
2A. Not sure what you mean by 18dBFS. Since the H9 has a soft clipper at its input, it is hard to judge its maximum conversion level (if this is what you mean).
2B. Ditto.
2C. Apologies for stating the obvious, but you do it by ear. Some effects are very dependent on the input material and the parameter settings, so there can be no hard and fast rules. If we were to make it totally clip-proof under all circumstances, the dynamic range would be severely limited. This applies to all effects, not just ours.
