Learn Hardcore Spanish (Latin America): Me encanta comer una quesadilla caliente con queso derretido. - I love eating a hot quesadilla with melted cheese.

The thing you love is actually the subject of the sentence.
The person who loves it is shown by an indirect object pronoun. Here, me means “to me.”
The verb agrees with what delights you. In “Me encanta comer una quesadilla…,” the entire action comer una quesadilla caliente con queso derretido is treated as a single idea (singular), so you use encanta.

Also, when you love doing something, you attach the infinitive directly—no de is needed (compare: Me encanta nadar, not Me encanta de nadar).

Why do we use una before quesadilla?

Una is the feminine singular indefinite article in Spanish. You use it because:

quesadilla ends in -a, which makes it a feminine noun.
You want to say “one (or a) quesadilla,” so you use una.

If you talk about more than one, you’d say unas quesadillas; for generic statements you can even omit the article and say Me encanta comer quesadillas…

Why do adjectives like caliente and derretido come after the noun?

In Spanish, the default position for most adjectives is after the noun they modify:

una quesadilla caliente (a hot quesadilla)
queso derretido (melted cheese)

You can sometimes move adjectives before the noun for emphasis or style, but after is the standard.

Is derretido here an adjective or a past participle?

In this sentence, derretido functions as an adjective describing the cheese’s state. Although it comes from the past participle of derretir, it’s used adjectivally, so it must agree in gender and number with queso (masculine, singular).

What does con do in this sentence?

Con is the preposition meaning with. It links the main food item to what it contains or is served alongside:
“una quesadilla caliente con queso derretido” = “a hot quesadilla with melted cheese.”

How would you say “I love eating hot quesadillas with melted cheese” in a plural or more general way?

You can simply pluralize the nouns and adjectives:
Me encanta comer quesadillas calientes con queso derretido.
If you want to imply “some quesadillas” rather than all hot quesadillas in existence, you could add an article:
Me encanta comer unas quesadillas calientes con queso derretido.

Is quesadilla a diminutive of queso because of the -illa ending?

Despite appearances, quesadilla isn’t a straightforward diminutive of queso. It names a specific dish (a tortilla folded around cheese) and comes from Nahuatl roots. In modern Spanish, you don’t use -illa here to mean “little cheese.” It’s just the established name of the food.