Learn Arabic (Modern Standard): العنب أكثر حلاوة من الليمون.

While Arabic does have a direct comparative word for sweet (أحلى), it is very common and natural to use أكثر (more) or أقل (less) followed by the noun form of the trait. This structure uses the Accusative of Specification (التمييز) to specify that the grapes are more specifically in terms of sweetness.

What case ending does حلاوة take when read aloud, and why?

It takes the accusative case, making it حلاوةً (halawatan). The Accusative of Specification (التمييز) always forces this specifying noun into the accusative case, which gets an -an sound here because the noun must be indefinite.

Why are العنب and الليمون translated as plural grapes and lemons when they lack plural endings?

These are collective nouns. Words like العنب refer to the fruit as a general category or crop, which translates naturally to a plural in English. If you wanted to single out one individual grape or lemon, you would add a taa marboota to the end to make it عنبة or ليمونة.

How does the preposition من work in this comparison?

Although من normally means from, it translates to than whenever it is used to connect two things being compared. The standard formula for comparisons is always the comparative word, followed by the specification noun, followed by من.