"Sounds/looks to me like the bottom of the slab is shrinking more/faster than the top. Could be your base mix has more water than the top, or whatever your pouring them out on (mold base) is too porous and stealing your water. This is causing…"
"Make your first backer coat I little looser and do a thin layer. Don't worry about building up any thickness, just worry about getting a good lamination with the mist coat (you can use a chip brush/compaction roller to assist). Let…"
"That's going to be a tough patch. I sometimes use epoxy putty mixed with raw pigment. It will give a better read for color. If you still have the mold, I'd cast again for redemption. Looks really good for a first…"
"I would do each in 3 pcs. The main body, and then the two inner walls separately. The seam will be totally out of site with the way your arch hangs in 4". Just glue it to the inner unfinished wall and good to go.