Learn Swedish: Gästen klagade på vistelsen, och denne krävde kompensation. - The guest complained about the stay, and this person demanded compensation.

No, because vilken usually refers back to the noun immediately before it. If you wrote ...vistelsen, vilken krävde kompensation, it would grammatically sound like the stay itself demanded compensation! By using och denne (and this person / the latter), the sentence clearly points back to the person involved, specifically Gästen.

Why is the word 'denne' used here instead of a normal pronoun like 'han' or 'hon'?

denne translates to 'this person' or 'the latter' and is highly formal. It is often used in written Swedish, such as in news or legal texts, to refer back to a recently mentioned person without specifying their gender, or simply to maintain a very objective and formal tone.

Why do we use the preposition 'på' after 'klagade'? Doesn't 'på' usually mean 'on'?

While på often means 'on' in physical contexts, verbs in Swedish have specific prepositions they naturally pair with. To 'complain about' something is always klaga på in Swedish. You cannot translate 'about' directly to om in this case.

What is the base form of 'vistelsen' and why does it just add an '-n' for the definite form?

The base word is en vistelse (a stay). Because it is an en-word that already ends in a vowel, you only need to add an -n to make it definite, creating vistelsen (the stay).