What is worse: Carbs, Wheat gluten, Rice, Corn, Mercury, Phosphorous?

While researching commercially available canned foods, I am discovering
that it is almost impossible to find a brand that avoids all things
touted in the nutrition literature as bad:

1. High % of calories from carbs. (E.g., Science Diet, good otherwise)
2. Presence of wheat gluten
3. Presence of rice
4. Presence of corn
5. Mercury (mainly seafood, which would be low in carbs otherwise)
6. Phosphorous.

OK, so here is a question for the truly informed: If I can't avoid all
of the above-listed baddies, and it seems I can't, then which ones are
most harmful and must be avoided, while others can be tolerated?

Needless to say, if you know of affordable commercial brands that meet
all criteria, please do name them!