If I launch from a hilltop down into a valley, will the actual RTH height be (launch altitude + preset RTH height) or otherwise?

If you set the RTH height at, say, 50 metres, that reference is in relation to the take-off point (or homepoint). So, yes.

already answered in post #2

Be carefull in your start point is close to the edge of a canyon, hill etc....
If you fly down and you loose contact....if the distance is too close to your HP if will fly at current height towards HP.....   ;-(

Same for taking off close to water....loosing contact it will land at position.

Read the manual for distances.

cheers
JJB