Learn English – A superlative form of right

Both of your examples

This is done easily.

This is done more easily than I thought.

are grammatically correct.

However, I would personally consider both sentences to be slightly awkward or incomplete, since you could express the same ideas using adjectives. If you had no more details to add, then more natural expressions might be

This is easy.

This task is easy.

This was easier than I thought.

This job was easier than I thought.

The original examples would also be fine if there were more details to complete the sentence, so that there is a reason to use the adverbial form rather than the simpler adjectives that I proposed above.

Calculating this antiderivative is easily done using integration by parts.

Calculating this antiderivative is more easily done using a trigonometric substitution.

Well, the opposite of low isn't up but high, which has the comparative higher and the superlative highest. So they're equivalent in that regard.

The opposite of up would be down. But up and down when used to describe relative directions are adverbs, not adjectives, and they don't have direct comparative and superlative forms (unlike say, badly, worse, and worst). While I don't see anything technically wrong with saying more/most up/down, that's not something you really hear in practice. Instead we can say something is farther up/down or farthest up/down.

Upper is a plain adjective, not a comparative. You can say: John is in upper management, but you wouldn't say John is upper than Jim. Also, while downer is a word, it's a slang noun and not an adjective. downest is generally not recognized as a word as far as I know, but neither is uppest.