I only survived with one life out of three on Insane, so it does have the difficulty down. However, I think it's for the wrong reasons, because the level didn't play much like the original Plutonia. It's very rare for Plutonia to overload the player with enemies (can't think of much other than Go 2 It), so most of its difficulty comes from placing the monsters in very tactical positions. I'm actually building a set with similar inspiration in tactical gameplay and balance, so I think it'd be fair to use slightly more difficult robots like Fox Attack Bots and provide light weapons like the Spreadfire early on in exchange for making it with less robots, but in worse positions.
The structure of the level was a nice start. Experimenting with cube structure more is always a good idea as you learn more about the editor, but I want to note there were a few cubes in the trapezoid halls that the editor would have flagged illegal even though Retro didn't actually seem to mind. As you start dabbling with warped cubes more and more, you'll want to check the diagnostics tool frequently to keep concave cubes or other geometry errors from slipping into the final product undetected.
I don't know if I'll have a level idea to spare between working on my own set and helping with Descent 1½, but I really like the concept behind this set and I'd be more than happy to give you any pointers to help you out. Feel free to message me or ask for any advice if you like.