The phrase "more busier" is redundant—it should just be "busier," which implies the "more" aspect already. As seems like an attractive choice, but the phrase "in demand as magazine editor" displays proper idiomatic usage.

We use the word "less" to describe a quantity that doesn't have discrete parts. For example, we can say that we have "less money," because money doesn't have an inherent discrete unit; however, we cannot say that we have "less dollars" because a dollar is a discrete unit of dollars. In the case of the sentence above, since ideas have a discrete unit of a single idea, we say "fewer ideas," not "less ideas."

When two subjects are being compared (such as the twins in this sentence), the appropriate preposition to use for comparison is "between." "Among" is only appropriate when three or more subjects are being compared.

When describing the quantity of an item that is countable, like "presents," "fewer" should be used. "Less" should only be used when a particular item is not countable, as in "less water" or "less fuel." 

This sentence uses a superlative incorrectly. Since there are only two options, “more” should be used instead of “most.” The corrected sentence reads, "Between Melville’s Moby Dick and Cervantes’ Don Quixote, I find the latter to be more compelling."

This sentence uses a superlative (“most”) when comparing between only two items. The comparative “more” is better here. The corrected sentence reads: Of English and biology, I find English to be the more challenging subject—nothing is logical!

This sentence uses a superlative adjective (“longest”) to make a comparison between only two items (the cheeseburger and the salad). Superlative adjectives are used to compare three or more items. A comparative adjective would be grammatically correct. The corrected sentence reads: "The mushroom-topped cheeseburger and the roasted beet salad both sounded appealing, but Henry decided that the cheeseburger was likely to keep him full longer."

This sentence uses a comparative adjective “bigger” when there are three elements being compared. With three or more elements, a superlative (in this case “biggest”) is required. The corrected sentence reads, "While my friends Jenny, Tom, and Beth all enjoy breaking into spontaneous song and dance routines, Beth gets the biggest kick out of it."

This sentence uses a superlative (“the most convenient”) that only compares two things. A comparative is more appropriate. The corrected sentence reads, "Richard debated endlessly between two options on the drug store shelf: the sour candies were fun because they made you pucker, but the lollipops were more convenient because they came on little white sticks."

This sentence uses a superlative (most) when it is only comparing two things. A comparative would be better. The corrected sentence reads, "The toddler could not decide which he liked more: chasing the dog around the house or throwing his spoon down the laundry chute."