If some units are under the ridge and some not how do you distinguish, for example, armor attacks which should be halved against the ridge units but not presumably against units on top. The armor is halved as long as units are under the ridge card. Again, if you are able to survive the arty round, you could have units outside and under the ridge card with little difference. One assumes the cohesion phase. Yes, at the beginning of the cohesion check phase.
Effects carry through movement and combat phases.
Units defend normally so no effect during other player's turn. OOS units are OOS for the turn. Recheck at start of next turn. Treat as an exception. Perhaps all this is balanced by the fact that the usefulness of the Von der Heydte card hangs on its proximity to the top of the deck.