Learn Finnish: Kuka vastaanottaa vieraat edustajan sijasta? - Who receives the guests instead of the representative?

The word sijasta is a postposition meaning instead of or in place of. Like most Finnish postpositions, it requires the word preceding it to be in the genitive case. So, instead of the representative is constructed as edustajan sijasta.

Is sijasta related to the noun sija? Could I use sijaan in this sentence?

Yes, sijasta is literally the elative form (out of) of the noun sija (place, position). You can also use the illative form sijaan to say edustajan sijaan. Both are extremely common and mean instead of in this context, so you can freely choose either.

Why is the object vieraat in the plural nominative form rather than the partitive plural vieraita?

This is a total object. The verb vastaanottaa (to receive) describes a completed or distinct action here, and we are talking about a specific, entire group of guests being received. For plural total objects, Finnish uses the nominative plural form. If the sentence meant receiving an unspecified amount of guests, you would use the partitive vieraita.

How does a compound verb like vastaanottaa conjugate? Do both parts change?

Only the second part, the base verb ottaa (to take), takes the conjugation endings. The first part, vastaan (towards/against), acts as an unchangeable prefix. For example, if you wanted to say I receive, it would simply be vastaanotan.