Storing Luggage in Stockholm

Swedish Rail has their own helpline. Why don't you try it?
https://kundo.se/org/sj/d/what-is-sjs-faq-and-how-does-it-work/

Both the train station and the attached bus terminal have lots of lockers available.

http://stockholmtourist.blogspot.com/2014/02/stockholm-central-train-station.html

Yes, the station has lockers. I'm not sure why, though, but the day I visited, all the lockers were in use by mid-day. I arrived fairly early, and had no problems, but when I returned to the station a few hours later, a line had formed for people waiting to get a locker. That people would form such a queue spontaneously says quite a bit about the Swedish national character.