Well, energy sensing in general, everyone trained to fight can sort of do, but just the very basics like telling that someone is a lot stronger or weaker. However energy sensing to figure out a more exact level of power or where a power is, like say what is required to use Instant Transmission when you have never been to the destination, is a trained ability. Therefore the former type would be Level 1 and the later Level 2. Utility based techniques are still techniques after all, not attacks but techniques.

I am intrigued by the idea of eating the flesh of your enemies to regrow your own, and/or perhaps even eating the flesh of others that have features you like and want to acquire. I think it is a great power for Envy, I personally see Deceit more as a master of illusions. I have to agree that using it for regeneration would still use up a fair amount of stamina, but would be more instantaneous and therefore more viable in battle.

Lastly, just some food for thought when designing the character, while the Arcosians are beings born really powerful and have never thought of training to get stronger, the Makaioshins are levels beyond that. They are gods of a realm of vicious demons, they have never had an equal (save for each other) and long ago grew bored with their realm. Ever since they have watched the mortal realm through their pool of fire and now are given a chance to be the director of that show they have been watching for so long. So now they are trying to each spread their own brand of evil in an entertaining way, both to themselves and to whatever servants may still be watching that pool of fire.