Firstly a eurail pass is a ticket, where trains are compulsory reservation you will also need to obtain reservations, which usually are less than €10 per seat.

Standalone tickets (which is how the vast majority of passengers buy tickets) are entirely separate and cannot be used with a eurail pass nor exchanged for one after buying.

Refunds on purchases are down to the terms of the individual ticket types and sellers.

Based on your plan, of the 4 countries you are planning for:

Denmark does not require reservations, this will apply to Malmo - Copenhagen and return.

Sweden has a mix but the fast trains Stock - Oslo and Kiruna - Stock will require reservations, Gothenburg  - (Malmo) - Copenhagen does not.

Finland is officially listed as reservations not mandatory but in practice travelling on their long distance routes without reservations is not a great idea as all regular tickets are reserved so you could end up being turfed out of seats.

Swedish train reservations can be booked for passholders on https://www.sj.se/en by adding interrail/eurail as discount. Be aware that it will also show bus connections that are not valid with eurail (when searching for passholder reservations the fees for the bus services will be high, these are in fact full fare tickets for the bus), as the line to/from Oslo is currently being worked on there are few trains and mostly buses.

Finnish trains can only be booked online through eurail booking service with tickets sent by post.

European timetables change in early December and Swedish trains are not currently open for booking beyond 11 Dec.

The journey from Kiruna to Stockholm with VY night train is already open for reservations after 10th of December at sj.se even though SJ hasn't released their timetable.

I have not been able to make any reservations for train travel for my upcoming trip in December and I have been trying for months. The train websites want full payment and even when I enter a EURAIL promotion code it still wants full payment. Based on conversations in this community I can not get reimbursed for purchased fares not associated with my Eurail Pass. All the night train cabins are booked from Helsinki to Kiruna and most of them are gone from Kiruna to Stockholm. Since there is no way to make an online train reservation I think I am going to get a refund on my pass and just fly everywhere. This is quite possibly the worst reservation system that I have ever encountered and I have traveled all over the world. Does anyone have any specific recommendations on how to secure reservations? I would like to book the Kiruna to Ostersund route in a private cabin for my daughter and I for 01 Jan. Also looking for the following reservations: Stockholm to Oslo on 19 Dec. Oslo to Gothenburg on 21 Dec. Gothenburg to Copenhagen on 22 Dec. Copenhagen to Stockholm on 24 Dec. I am getting desperate and could use some help. The Eurail App is useless since it doesn’t have any options for my dates.

That depends on the conditions of the tickets you bought.

There is no direct night train Helsinki - Kiruna. What exact trains would you like to use?

What date for Kiruna to Stockholm?

Kiruna to Stockholm can be booked online on sj.se, as shown above. You need to select the Interrail/Eurail discount and enter any number consisting of I or E and 8 digits (it's not printed on the reservation).

Kiruna to Östersund on 01 Jan.: I'm not sure if you want to get up at 5 in the morning and wait for a connecting train. What exact train(s) were you looking at?

Stockholm to Oslo on 19 Dec.: Booking opens on 9 November, via sj.se.

Oslo to Gothenburg on 21 Dec.: Reservations are optional. Can be booked by phone via Entur (Add +47 for Norway). Reservations will be sent via email.

Gothenburg to Copenhagen on 22 Dec.: the hourly direct regional trains don't need reservations.

Copenhagen to Stockholm on 24 Dec.: Booking opens on 9 November, via sj.se.

The Eurail app is not reliable enough for planning and is still incomplete for after the timetable change on 11 December.

In addition, as has been said here and in many other topics, not all reservations for after the timetable change on 11 December are open yet, including many reservations in Sweden.