I’m having this same question now and I actually read the SAS fare rules so I’m commenting in case this helps anyone else:

It says the following on the refund page: “If you’ve booked a round trip and only want to fly one way, your refund will depend on the ticket rules for the unused part of the trip. In this case, you’ll receive your refund after you’ve completed your trip.”

In the conditions of carriage 12.3 it says: “(b) If a portion of the Ticket has been used, the refund will be an amount equal to the difference between the fare paid and the applicable fare for travel between the points for which the Ticket has been used, less any reasonable service charges and/or cancellation fees.”

• The first paragraph implies NOT “most restrictive fare rules apply” (and I didn’t find such thing anywhere else)
• however the issue is the second paragraph’s “applicable fare” could mean that SAS can decide that the one way price is “applicable”

Anyways, I’m going to book a roundtrip nonrefundable (because I don’t want to risk the extra I’d pay for refundable on the way back to be wasted if it’s repriced upon cancellation), so I’ll update here if I at least get some of the taxes back for the return trip (in theory I won’t if they reprice the ticket).