Hallo, I count the traveldays and I see 5: 2 to get there and 3 to come back. IN a period of 1 month you can choose 5 what is called traveldays-on such days (must be marked either on an old-style paperpass or automatic on a new style app on the fone pass) you can board as many trains that accept InterRail as you can manage-and even finish the trip next day on an overnight train. Prices can easily be found on this site-they differ for junior/adult/senior. You can choose 1st or 2nd-but in fact many trains in SE do not eve have 1st anymore, so thats useless.

BUT you must also make the REServations for all trains that need to: that means currently IN DK due to covid-can be done online if you manage to understand danish-and certainly In SE and it can only partly be done online. For the overnite train STO-Norrbotten (on to Narvik): very often ,as this is main ski-season for the Swedes, you will not find sleepers all the way to Abisko/Turiststasionen, but more often as far as Boden, where a portion splits off to go Lulea. You will also need to comply with all the different rules re covid/wearing masks or even making online forms. These can change at very short notice to.

I did not check times; but be aware that esp. in Germany on DB trains are very often VERY late and this may mean missed connections-quite nasty as there is only this only train overnight from STO to beyond Kiruna. It would be very prudent to start as early as you can in the day from Wallonie and try to reach at least Hamburg as quick as you can. ICE trains in DE do not need REServation-but you can if wanted for 4 eur/ride on bahn.de. This also means you can stock up on much cheaper food for very expensive Scandinavie further on. Plus due to covid/omicron there are also many reports that trains simply are cancelled as too many trainstaff is sick or has to remain isolated. But thats already the case on SNCB too.

Bonjour et bonnes voyages!