Hi Curtley
It seems to me that this has fallen through and they are just giving you an excuse. It could be that the husband and wife are in disagreement about what they should do. So they are unlikely to be interested in any complicated deal.
What you are proposing is too complicated.
A much simpler solution would be to suggest to them, a deferred closing date. In other words, you would exchange contracts now but then close the sale in March 2022.
Both of you would be legally bound by that.
The downside for you would be if anything happened in the meantime, and you could not get a mortgage, you would, at the very least, lose your deposit.
On reflection, the best approach for you would be to move on. Tell the estate agent that you understand and you will now look at other houses. If this comes back on the market, he should let you know.
Brendan