RTH advisory / warning aren't an issue unless RTH is happening
What's causing this ?
Or are you simply sick of the advisory playing when you have triggered this ?
Cliff dangers, all depends on direct flight plans.
Best to obviously (and safely !) get close enough to the cliff edge to get the shots you want, then you need to stay *reasonably* distant to the cliff with the MPP to keep good signal . . . not only to the controller, but also for good satellite connections.
Just keep an eye on the signal / sat meters on the screen, and adjust flight if needed to keep control.
It can be dangerous to let RTH happen in those situations, even simple flight . . . cliffs notoriously have some big wind patterns / updrafts, and if an offshore wind, low pressure systems near them . . . they can suck your drone in.
Stay in P mode to keep sensors working, the front OA might help if this happens, as long as the MPP is facing the cliff, and ground OA can help if descending and a crag or something is sticking out, but that's probably too close !
Post up the video, hopefully those big coastal cliffs you have around the place