Can I nullify the return trigger with MEV nullifier?

I don't think so, as I don't think it's a trigger. I'm not sure, though. It has the phrase "at the start of", but not at the beginning, and just seems to be clarifying when. I'd love to be definitive, but I can't. As it is, I don't think it's a triggered effect that goes off. If so, though, then MEX Nullifier should be able to negate it.

Bodyslide's return uses a delayed triggered modifier---essentially, the initial Bodyslide effect sets up a floating triggered power.And as for "opposing," the controller of the triggered effect traces back to the controller of the initial Bodyslide effect.So, yes, Ultimate Nullifier can stop Bodyslide's return.