So as I suspected, the reduced service on the PRN-DUS route was by no means caused by extraordinary circumstances, but was a profitability-based business decision due to the pandemic (not confirmed, but obvious to me)
I got the standard compensation (EUR 400) with the help of the Kosovan civil aviation department, and then reimbursement of a "lost" rail ticket (GBP 10.55) directly via Eurowings. The Kosovan body, while slow, seems to know their stuff, so kudos to them for that!
So for future reference, the common belief that any and all cancellations remotely having anything to do with the pandemic is ineligible for compensation is false!
On 5 July I emailed Eurowings (firstname.lastname@example.org) requesting compensation.
On 20 July I got the much-expected refusal claiming extraordinary circumstances without specification or evidence.
On 22 July I e-mailed the Kosovan civil aviation department (email@example.com) attaching 1) Eurowings's refusal plus a translation to English; 2) the original and modified itineraries; 3) the "lost" rail ticket confirmation (GBP 10.55)
On 28 July the Kosovan civil aviation department e-mailed Eurowings (with a copy to myself) asking for a statement regarding the cancellation.
On 27 September I asked how things are coming along.
On 28 September the Kosovan civil aviation department asked me to attach the itineraries again "so that [they] can proceed on this case based on EC regulation 261/2004", since the 6-week deadline had passed for Eurowings to respond.
On 29 September I emailed the itineraries again as asked.
On 21 October I asked how things are going.
On 24 October the Kosovan civil aviation department told me Eurowings had backed down and would grant compensation but needed my bank details and the rail ticket confirmation.
On 25 October I provided these details.
On 7 November the Kosovan civil aviation department forwarded an email from Eurowings saying they would pay out EUR 400. When I asked why they wouldn't reimburse the rail ticket, they referred me to Eurowings, closing the case on their end.
On 26 November I received EUR 400 on my account. The same day I emailed Eurowings asking why they wouldn't reimburse my rail ticket and explaining why they should (had it been in the opposite direction, they would've been legally obliged to, and the price is the same)
On 2 December Eurowings told me they'd reimburse the rail ticket (in EUR)
On 22 December I received EUR 11.60 on my account.