MS support suggested (off the record) that I find someone who had a Surface Pro still in warranty and ask them to ask for a replacement power adapter. Said that

MS support suggested (off the record) that I find someone who had a Surface Pro still in warranty and ask them to ask for a replacement power adapter. Said that they'd get one free of charge, no questions ask. Unfortunately I don't know other people who own a Surface Pro so that didn't work for me. Anecdotally I've heard of Apple offering informal service outside of warranty, so was hoping I'd get something similar as it felt like a borderline case, but no luck in my case. Even a reduced price to replace the faulty accessory would have gone a long way to improve the support experience for me. In the end I got a half-hearted apology from them and that was it.