It was still wet in the back because you layered it on too thick. I did that the first time I used it myself. Unfortunately, you ARE going to get some shrinkage. IMO, it actually makes the BG look more natural, however, if there was any "wet" (read uncured or even slightly soft) GS in the area where you carved out your pools, you are probably going to have a little bit of a problem. The shape of the pools may change, quite a bit, and you may have pools quite smaller than you intended. I suggest you wait a couple of days before covering the GS to see how much everything shrinks so you can tell if you need to add more GS or do something else to get the look you desire. Of course, it may not shrink at all, but I think that is highly unlikely. Next time, unless you want the shrinkage effect (I did it that way on the BG I just made for my snake so it would shrink some), just make smaller layers and allow for at least partial curing between layers. You could also carve some polystyrene for the pools and stick it in the GS. If you did that, there would be no possibility for shrinkage, at least for the pools. Hmmmm, that's a good idea I just had, lol... I'll have to use that in my CWD vive.
Other than that, it looks great! Keep up the good work and keep us posted!