Hello,
What your accountant is telling you about having to repay the VAT (pro-rata) is correct, as I understand it.
In truth, I think just about anyone I have ever spoken to who bought a French sale and leaseback, feels they were very badly mislead and subsequently when things went wrong (as they almost always did), there was no support from the French authorities etc. As such, you are probably in very good company, in terms of how you are feeling about the experience.
That said, I would expect the French tax authorities to take legal action against you and if necessary, have the results of any French judgement they might obtain against you, enforced here also. Remember they don't need to worry about who is paying the legal bill, so they have nothing to lose by chasing you for the debt !
I am actually surprised you managed to get your sale proceeds, without the payment for vat being retained by your French lawyer.
As an aside, I am fairly sure you can offset your Capital Loss on this investment against future Capital Gains (here in Ireland) if you are lucky enough to have any.