Don't they have adverse possession laws that would clear the land rights after a decade or so ? (UK and most countries have, I have expected HK to have to)
I'm not sure about adverse possession, but my understanding is that the church behind Cheung Kong Centre is the only place in Hong Kong that privately owns land rather than leasing land from the government for 99 years. It would be better to sort out efficient use of land before the leases need renewal, but at least there's a mechanism to reclaim truly unused land.
No, in these cases the right to build one house lasts for the duration of one male heir since 1972. There are also restrictions on the conditions of sale if I’m not mistaken, which makes it more difficult to sell off to big developers.