” if the Buyer had some cash flow problems, then they could choose to let a charge go against the property”,
Yes, it can. It may also that he never takes the charge off & pass it to next buyer. Knowing how chaotic the process is the new buyer will end up paying. As it has happened in many cases to foreign buyers.
Would the Seller be left in limbo, trying to claim back a payment that hadn’t been made?
Yes, He/She would be. barring the current situation most of the people would have made a profit on their property so once all the cost, tapering relief is allowed the situation would have even it self out. I AM NOT SAYING THIS WILL HAPPEN IN ALL INSTANCES, THIS IS JUST A DIFFERENT ANGLE ON THE SUBJECT.
At the least, it wouldn’t make things easier.