UA refunded non-refundable international return flight today via customer care phone call with no problems. Even got through immediately.
Dulles to Toulouse, Fr, departing 16 April, returning 6 May. At least one of four flight legs cancelled by UA. Rep offered me credits/vouchers and I simply said "refund to my original form of payment (credit card) please". She went off to check with supervisor and a few minutes later came back to indicate full refund was being processed. 5-7 business days to see credit to my account.
Within minutes this showed up under My Trips on my account to confirm cancellation and refund:
Ticket status: This reservation has been canceled and refunded/voided as applicable.