Learn Swedish: Vissa marmelader är dock väldigt söta. - Certain marmalades are however very sweet.

This is a concept known as sortpluralis (plural of kinds). When you pluralize an uncountable mass noun like marmelad (marmalade/jam), mjölk (milk), or ost (cheese), you are referring to "types of" or "portions of" that substance. In this sentence, marmelader specifically means "types or brands of marmalade."

Because marmelad is an uncountable mass noun, shouldn't the adjective just be the en-word form söt?

If you were talking about marmalade in general as an uncountable mass ("Marmelad är söt"), yes. However, because we are using the sortpluralis to talk about specific types of marmalade, the noun is grammatically plural (marmelader). The adjective must agree with this plural noun, so it takes the plural -a ending to become söta.

Could I use några instead of vissa here?

You could, but it changes the nuance. Några marmelader just means "some marmalades" (referring to an indefinite quantity or a random few). Vissa marmelader means "certain marmalades" or "particular marmalades," which emphasizes that specific varieties share this trait of being very sweet.

In English, "however" often starts the sentence. Why is dock placed after the verb here?