This seems like an interaction that wasn't planned for. When Celestial Tribunal came out, we didn't have any heroes that could manipulate variants. With Completionist Guise out, I'd say Celestial Tribunal will continue to work as it was designed to.
Now, the way I would literally interpret the interaction is that the new variant would act as though the game started, and its maximum HP would be the HP printed on the card. Representative of Earth is worded in a way that leads me to believe that "setting its current HP to 10" is an action that happens once. It doesn't seem to be a constant check. If it were, I'd imagine it would read "That hero's maximum HP is now 10." In that way, it feel more like a game rule, and not a one time action. I agree with Skippy though, the next sentence "If that hero is incapacitated, the Earth is sentenced to destruction. Game over." is tied to that hero altogether, and no change in variant would matter.
I would personally rule in favor of the former scenario. It would still be 10, because that's how it would work as intended.