1. The deconstruction planner filter page should list cliffs, so they can be white/black listed.
2. Shift-Lclicking to place a blueprint should destroy any cliffs in its path, similarly to how trees and rocks are currently removed.

There is a landmine icon in the deconstruction planner; it perhaps would be better to use the cliff explosive icon instead?

I'm reasonably certain that point 2 also already works as requested.

Sorry for not replying, I've been busy the last few months.

1. The cliff explosive proxy does appear in the white list (and it's got the right icon now), but it doesn't do anything. The deconstruction planner setup isn't passed in with the event so I can't check it easily, but there might be another way to get it, I'll have a look when I get a chance.
2. I'm not sure it's possible to change the behaviour of blueprint placement from a mod like this (at least without changing the cliff prototype which I'd rather avoid). If you know of any mods that do, say and I'll have a look how they do it.