The problem isn’t just DfT rules but also Immigration & Customs for the CTA.
1) all pax must clear Immigration at first point of arrival into the CTA ie EDI will clear for Eire & DUB would clear for U.K. this is where the problem arises is that for Immigration dirty passengers will be mixed with clean passengers. So a pax who is Transitting without Visa through Dublin will then be on a DUB - EDI CTA which increases the chances of undocumented arrivals. In practice a CHN national with a schengen visa could fly from Paris to DUB to join the DUB-EDI-PEK flight without see the Garda as long as they Transit airside. They then get off the flight in Edinburgh and claim Asylum. This is why these flights require permission from the Director General of Border Force
2) although CTA is free movement of people it’s still a Customs area for Prohibited & Restricted goods. Again the risks are higher for these flights.
3) nobody knows what will happen regards the U.K. & EU Border that’s being created by Brexit
4) Edinburgh had also asked for the passengers to enter the common departure lounge whilst being processed to DfT standards (they want them buying Duty Free) again this is a problem with an exit to the U.K. that’s not controlled.
Very complicated hence a great deal of negotiations. Also aren’t Hainan in finincial trouble??