The damage dampening across walls would be really challenging to balance considering not all civs have a full siege capability. We already sort of have a version of this regarding the units atop stone walls, only its level of exploitation is much more controllable.
About palisade walls I agree they should be used not for slowing down pathfinding, but by blocking it altogether (closing off sections) like walls are supposed to do. palisades are very bad at stopping melee cavalry (in particular axe and sword), so I think melee cav (not sword/axe inf) should have a 0.3x multiplier against palisades. If they are too effective at stopping all aggression, then we could consider a cost increase or some other action. nerfing melee cav vs walls would also give some advantages to using infantry rushes which we rarely see in MP anymore.
Ease of placement improvements would also work wonders for palisades and stone walls. It may look ugly, but increasing the allowable overlap would help a lot especially for straggler trees, small mines, and other res and terrain.