This whole situation does sound most peculiar! It sounds like a “damned if you do and damned if you don’t” case. It sounds like the lawyer wants to proceed regardless of any claims you might have to make in the future, to get the main sale completed. But it seems to me that once all parties get all their money, they tend to wave bye bye and leave you to pick up the mess they left behind. Four years is an inconceivable amount of time to be waiting on this anyway, but I personally wouldnt hand over another penny if i was unhappy thus far.
I know for example that one of my neighbours refuses to complete his sale now, on the basis that his house is not up to spec, even though pressure is thrown at him to cough up. This is even before a habitation licence is issued. They want him to join the miserable fold of our street. But of course their not telling him the whole story, the developers keep on telling him all will be resolved manana! Not true, manana has lasted seven months.