For one, I misunderstood you: you must refer to an overlay that applies to the whole map like what happens on Scen 3A. I thought you were talking about the neighbourhood around thrones (strategical feature or site), and the scenery and tiles that we could find if we enter such a site in tactical mode. That why I mentioned a mountain background hex (exactly like the frozen cascade in Scen 3A (UG, to the west)), and a reef island (exactly like some water sites) on the strategical map. As for the tactical map, it could be a cavern scenery along with wet tiles for Mab, and a drown temple for Nimue.
For two, as regard to the shifting overlays, I wouldn't mind having a patch of terrain not as visible as a volcanic or a snowy climate overlay. Remember that SR is for the experienced player! However, if I can see that an overlay can be changed and changed again without altering the map too much (terrain staying unchanged), I fear that the game can't remember the ancient terrain before it is changed by some 'Path of' ability (turning the land into dense vegetation or barrens) or some spells such as "Drench the land". I mean, these changes don't affect a pure overlay. We would end with a marshy zone even after Nimue looses her hold on the land.
Now, if the game can remember, I still think that (Wetland + Temperate) would suit Nimue best. About (Wetland +) Noxious for Mab, I'm a bit reluctant, because Mab is not the mistress of poison and decay. She's associated with the underground, its riches, and its denizens, even if Goblins do happen to be both immune and crafty when it comes to poisons and diseases. By the way, in AoW2, poison was a Water spell. Also, if you want something impressive, you could get inspired by AoW2 Drench the land: as time passed, low-level hexes turned into water pools, and see banks ate inland. Prehaps add a site that grants Basic Navigation!