Thank you for the additional explanation! It is very important to view any advice through the lens of your personal goal. What you want to achieve is all that matters!
I feel like I am hijacking your thread which I don't mean to. Please feel free to completely ignore my advice. I honestly am just trying to let you know what I wish someone had told me. Please udog, Mr Mold Maker, or anyone else correct me on any of this! It helps me learn!
Cast in parts/single cast -I would go with a single cast for the entire mask, but start by small tests. Print a small test mold of an area only having one antenna and maybe 8 frills plus 3 or 4 inches of connecting skin. Add a small bleeder hole for the antenna. Mix a small batch of silicone, color it, use a syringe to fill the frills and antenna, then use a toothpick to release any air in the frills or antenna. let that gel 15/ 20 then do another small batch of silicone and fill the connecting skin. This would replicate the steps for a real casting and let you know about possible problems. This can make sure the frills and antenna cast well and release without tearing. you could also use this mold to test coloring and different silicones. cast one with power mesh on the back and see the difference. Test one completely opaque and one making use of deeper colors and translucency, pick the one you like! I really wish I had started with smaller mold tests. I literally spent thousands of dollars to learn what I could have with under $100.
Dragon Skin/ Ecoflex - I vote 100% Ecoflex. If I was casting this, I would try Ecoflex 00-50 for the frills, antenna, and some of the larger bumps, then pour the main skin in Ecoflex 00-30 with a power mesh backing. Possibly even Ecoflex 00-20. Degass the silicone! I bought a small cheap degassing chamber on Amazon that works fine.
slit up back -this is completely your call in terms of style or accuracy to original. I would not, personally add a slit in the back. If you do, absolutely use power mesh! Once silicone is cut or starts to tear, power mesh is the only thing I know that will stop the tearing. If I put a cut in Dragon Skin 30, I can easily tear more from that point. I have not had issues casting with power mesh, but that is just me. I buy power mesh through Reynolds. If you cast in Dragon Skin, I think you will definitely need a slit up the back to be able to pull the mask on.
Seeing my advice, I am going to try test printing a small mold from my ZBrush model and test casting.
Please post pictures of the molds you print!