The Mercury etc. connection seems a bit far-fetched. It would be more logical if Neptune and Poseidon were swapped, so that the Mercury group could be planets or Roman gods.

Really clever, and great ideas, but doesn't work as there are multiple solutions to the grid, within the parameters of your group links.Neptune and Poseidon can be swapped between groups, as both are mythical figures in film titles and classical gods of the sea. Jupiter and Mars are both Disney characters, so could be swapped from the film titles group, with Pluto in the Disney character group as there are at least two films with Pluto in the title.