What a great building to take inspiration from! Stairs, arches, can't go wrong.
Sorry you are having problems with the plaster of paris, I haven't used dental stone so I can't tell you the difference.
Two things I use to fill in gaps or bubble holes:
Rub two discarded blocks together to make some plaster dust and then put glue in the cracks between blocks and then press fill with the plaster dust - looks kinda like mortar.
For big gaps or more consistent filling, I use this:
In fact, if you look at my gondorian wall thread, you can see where I used this. I had built the tower in two pieces and they didn't quite line up when I put them together. I used the putty to even it out. I pressed a block into the putty to give it the same texture, what I didn't do and should have done is carved block lines into it.
Will be following this - looks great!