The only fitting I am going to take precaution on is the water fitting. Although the 3in. pipe will only be sitting within the first 3in. of water. A leak will not loose much, and any stray fish should have the chance to scoot out, or flop their way back. Any newts should be able to climb back into the drained area if they need to get out of the water.
The junction will be a standard flex hose, that will be hidden from view by the frame trimming, so a little bit of motion will not harm the connection. Where as the upper connection, to the terrarium, will be a loose fit pipe. The loose fit pipe will be secured with normal window trim foam tape, and additionally will be hidden by a rock cave entrance and the between trimming.
The gap that remains at the back side of the middle tanks, where it sits in the corner should be enough to stand in, if not, I will make it enough to stand in. Additionally, the top tank will rest 1/3 on the aquarium below, and the other half will be supported via an extension from the aquarium stand. Not directly on the aquarium rim, there will be a 1in. gap between the two, which will fit the modified 3/4in. frame that will support the above tank. Mounted directly to the 3/4in. frame will be the hinge for the aquarium lid, to provide access for feeding and cleaning.
You can see the thin gap in the last drawing, the side view. Once again, these were not the best diagrams. I have a great imagination and what I see in my head is rather difficult to interpret onto a flat image.