You don't need to book a seat on that train. It is possible in some of the coaches but IMO it's a waste of money. Just hop on.

Additionally, the rail planner app and the Interrail website are often not up to date, so they should not be used to plan.

It may be done here: https://www.sj.se/en/ for SEK 34.

But there's no need to reserve. Most seats are unreserved anyway. 

Regarding the timetable: Don't trust the interrail app.

From CPH till MalmÖ-via airpot and tunnelbridge its 3/hr, ev 20 mins. The 2 of 3 continue till Helsingborg (has still ferry till Danish side-payable now for you) and 1/hr runs on via Halmstad till Göteb. So its also easy to stop off somewhere en-route and have quick look. One of the wodners of trains vs airplanes