The issue here is China hold all the cards with Hong Kong. Where does all the water, and power comes from? The mainland. The PRC, that's who. All Beijing have to do is cut the power, cut the water, cut the food supply, and then play the waiting game as the British and the US deals with a massive humanitarian crisis.
And...the British did not care about Hong Kong. Even after OTL Tiananmen Square, London kept a law that denied Hong Kongers full citizenship rights including the right of residency in the United Kingdom. The UK did everything in it power to keep the Hong Kongers out while everyone else was attracting Hong Kong immigrants as 97 roll around. (
Canada and the USA have more Hongkongers then the UK.)
Hong Kong is almost in the same boat of Aden: A similar port city that the UK abandoned without caring for its inhabitants in 1963.
As for Hong Kong the sunset clause was for new territory not the whole colony so technically Britain had to return part of Hong Kong but not all of it.
Is it even viable without the new territories ?
It is logistically impossible to actually hold onto Hong Kong without the New Territories, so no.
As for Macau...Portugal does not what colonies anymore. Portugal in fact offer to give Macau back a couple of times. (1967, and 1974.) They
wanted to get rid of Macau.