Learn Swedish: Veganen köper inga produkter som kanske innehåller laktos. - The vegan buys no products that maybe contain lactose.

This is because som kanske innehåller laktos is a subordinate clause (introduced by som). In Swedish subordinate clauses, sentence adverbs like kanske, inte, and alltid must go before the conjugated verb. This follows the BIFF rule: I Bisats kommer Inte Före Finita verbet (In a subordinate clause, the sentence adverb comes before the finite verb).

How would the word order change if "Maybe they contain lactose" was a normal standalone sentence?

In a main clause, the verb has to be in the second position (the V2 rule). So you would say De innehåller kanske laktos (They contain maybe lactose) or Kanske innehåller de laktos. Notice how kanske comes after the verb in the first example, unlike in our subordinate clause where the BIFF rule forces it to come before.

Could I say köper inte några produkter instead of köper inga produkter?

Yes, absolutely! Köper inga produkter means "buys no products", while köper inte några produkter means "does not buy any products". Both are perfectly natural and mean the exact same thing, but using inga is often a bit shorter and very common in everyday Swedish.

What role is som playing in this sentence?