Don't know if this helps or not @inkblink, but, our house is currently being "monitored for movement" , having noticed some internal cracks ourselves.
We paid for a building engineer/surveyor to come and give us his opinion, and he said he thought we had some degree of subsidence and to pass the issue on to our insurers.
Finding the cause of the movement is the main thing and then preventing the issue worsening is the next.
I felt overwhelmed too at the start but I have to say, our insurers, up to now, have been excellent.
We're in the process of having 4 months worth of movement readings/measurements being done and then they will deliver their verdict of whether this is an ongoing issue (sincerely hope not) or if it is historical.
You have my sympathy, as you have the added problem of trying to sell which we don't - yet.
As others have intimated, your property will be difficult to sell with this issue on going - or you may have to practically give it away, to get someone to take it on.
If your circumstances allow, personally, I'd take it off the market if your buyer does drop out, and get the issue sorted, then remarket it - and the price will have to reflect this has happened, even if it's sorted out.
I feel your pain.
Good luck with it.