Seeing the experiments you're producing is pretty exciting. It really validates what I'm doing when other people are able to pick up one of my designs and run with it. I think I might be able to help you with some of the flaws you're seeing in the castings.
You should try casting the cores in a neutral colored jewelry wax. I'm using this aqua wax
from Freeman which melts at a relatively low temp, doesn't tint the final silicone skin, and is relatively cheap. Parafin is a pretty pure wax which makes it pretty weak and a bad option for long structural stuff like the tentacle cores. Jewelry waxes like the one I'm using have a plastic component to them, which makes them terrible for turning into candles but super strong and rigid.
When I cast the tentacle cores, I wipe the hot wax off the surface of the mold with a flexible sheet of plastic or a bit of cardboard so I don't have to trim the flashing off afterwards. It just saves extra work and hassle later.
I assemble the cores with the help of an alignment jig
and a hot knife
for melting extra wax into the join between the base and the long core elements. The hot knife is just a sculpting tool jammed into a soldering iron with some steel wire to close the gap. You might get some sticky wax
to make the process of joining the base to the cores easier.
I still haven't found a way to get all the wax out cleanly. There always seems to be a little skin stuck to my robots no matter what I do. I've been considering getting a washing machine and dryer just for tumbling robots and squeezing out the wax, but it might be a matter of finding the right cleaners. The best process has been heating the silicone casting in the oven until the wax is liquid, squeezing out the wax with water resistant heat proof gloves
and then repeating the process a few times with soapy water.
You're doing awesome work and I'm eager to see what the next round of experiments look like.