Not to get too deep into the weeds, but all flights departing an EC member state are covered, regardless of carrier. So FRA-SIN / MXP-SIN / etc. would all be covered. However, it does appear that in the mishmash of illogical EC.261 court findings, a codeshare flight to the EC with a non-Community carrier are not covered, so SIN-FRA / SIN-MXP / etc. are not covered. However, for the specific scenario being discussed here, where a customer is denied boarding due, essentially, to a ticketing failure, I'd think that a case could be made against the ticketing airline. But it's definitely not as simple as "buy a full-fare ticket and sue," because there's a definite risk of losing the suit.