I'm pleased to close the loop on my experience posted about
upthread. After SQ telling me it was going to take ~4-5 months from cancellation to get a refund (they said 3-4 months more when I chased them almost a month after I had cancelled), I filed a chargeback with Amex, noting that SQ was refusing to provide my full refund (a) in a timely fashion and (b) specifically in contravention of EU261's timeline for refunds (ticket was MXP-SIN round trip, so EU261 was applicable). Amex has now completed its investigation and resolved it in my favour. I'm glad to be done with this.