One example of a non-compressible fluid is brake fluid. The brake fluid does not compress when you use your brakes in the car. Be glad that it does or we would all have problems stopping our cars. All hydraulic systems act this way. Water acts the same, too. Most fluids act this way.
Generally speaking, solids and fluids are not compressible. When pressurized they may distort, but they do not change in volume to any significant degree to cause issues here.
The key is not to allow the RTV to distort and therefore not giving it somewhere to go when pressure is applied. If you had the aluminum cavity completely constraining the RTV mold improperly designed allowing the plastic to run out, you allowed the RTV somewhere to go. There was a space in your aluminum constraint design allowing the RTV to distort and it allowed the plastic to run out.
Machine two aluminum blocks with a cavity with about a 3 mm void around the printed pattern. Put the pattern into the aluminum blocks and fill that 3 mm space with RTV. Hardness of the RTV (or urethane for that matter) does not matter. After curing remove the RTV and split it to remove the pattern. Re-assemble the cured RTV into the blocks and inject plastic into the void left by the pattern in the usual way. Remove plastic piece and repeat.
I cannot legally show photos here because of the confidential nature of our work. I have given all the information I can. If designed properly, this really works. If you understand how fluids and solids act under pressure, you can build the mold properly. I can try to answer questions, but I cannot design the system for anyone. Do not be put off by the science. Just make it so the RTV is properly constrained by the aluminum parts.