Yes and no, IIRC they were treated as British subjects but didn't receive full citizenship or rights. Over the years things changed, and right towards the end the government introduced a new class to essentially stop large numbers of locals from moving to the UK after the handover. Not exactly our finest hour.
@Sargon could tell you some tales about the joys of British nationality law.
This does seem like the easiest option for the UK.
Never going to happen. The lease on the New Territories was up—with Hong Kong Island and Kowloon being non-viable without them—so legally they wouldn't have a leg to stand on if they tried to do so unilaterally, and no Chinese government Nationalist or Communist would agree to ceding it. Even if Britain hadn't felt that good relations with China were more important than retaining the colony all the Chinese would have to do was their own version of the Green March and it would have fallen into their lap whilst Britain looked the villain of the piece.
Oh, I certainly can. There was a new passport category called
British National (Overseas) created. The passports look almost exactly the same as a British passport. When my wife went to open a bank account in the UK they assumed it WAS one until we pointed out it wasn't. Lucky we are honest.
This visa category is rather strange. Only Hongkongers born before the handover date can obtain one. It allows the holder to vote in all UK and UK-based EU elections, they can work for the civil service and they can stand as a candidate in elections. However, it does not allow the right of abode, and allows a person to stay for 6 months before they have to leave and presumably renew their stay.
This poses the question of if a holder can stand and be elected for Parliament, yet not have the right to live in the UK, then how does that add up? Well, you might say the Home Office would be, erm, pardon the pun, accommodating if that ever happened by fast-tracking the immigration process. Don't bet on it given the ramshackle nature of it being able to process stuff in a competent and timely fashion. As no
BN(O) holder has made use of this perk and ever stood and won an election, I guess we're not going to find out the answer to this interesting question unless a holder does.
The real reason of course,, was to make it
look like the British government was offering something to Hongkongers without wanting to face the issue of granting right of abode, and then have a lot of Hongkongers suddenly move to the UK. It is doubtful that would have happened. Sure, you would have seen an increase in people arriving, but no mass exodus.
Many people in Hong Kong, apart from having jobs, property and ways of life there, have family just over the border in mainland China, and probably would not wish to uproot unless things were looking to become really dire. Many wanted the insurance policy of a "just in case" option should things pan out badly in Hong Kong with China's administration of the place. You might say that is approaching in a way now, what with the recent incidents of booksellers being kidnapped, worries over PRC interference in HK's laws, and the general handling China has been shown to engage in while running the place. So maybe over the last few years, there would indeed have been a larger increase had a right of abode existed. There's probably going to be some effects and butterflies from such a thing.
Incidentally, the passports are not cheap and cost a fortune to renew. Another reason many Hongkongers were pretty angry about the whole thing.
Also, given they can vote, assuming they did have the right of abode,
they could affect the totals in such things as the referendum so there's a "what if" for you if you have all these new voters here, which way they could vote.
As for Hong Kong. Allow me to quote myself from another thread:
Incidentally, there have been at least two times in history when the New Territories might have been acquired in perpetuity. One was during the 1898 negotiations itself where the 99 year lease was almost casually fixed as a figure, and the second was in 1909 when Sir Frederick Lugard, Governor of Hong Kong, floated the idea of returning the British concession of Weihaiwai to China in exchange for the permanent acquisition of the New Territories. However, the Colonial Office didn't like the idea and binned it. Weihaiwei was returned to China in 1930. As part of such negotiations it may have provided an opportunity to address the status of the enclave of Kowloon Walled City as well.
Also, things get even weirder when you consider the proposal to create a "new" Hong Kong in Northern Ireland:
UK officials discussed resettling 5.5m Hong Kong Chinese in Northern Ireland
Archives reveal debate in 1983 over bizarre idea of moving millions of Chinese to Northern Ireland at height of Troubles ahead of colony’s handover to Beijing
Right, I think I've delivered the goods for this post. Have fun. 
Sargon
