Hi Emma - I'll do my best but must tread carefully as the specific applications are ongoing
If the Option Agreement exists and the T&Cs still apply then I would assume that they can still protect it, yes.
The devil will be in the details re the agreement/option fee etc but that's where any legal advice must come in. When it was drawn up and entered into would have been the best time to have that clarified including what might alter the legal situation over time- we would have no role to play in that.
A UN1 is a means of protecting such third party interests on the register -
Notices, restrictions and protection of third-party interests (PG19) - GOV.UK
The Tribunal is there to try and help resolve such matters and without the need to perhaps go to court but that will have been explained in the 'process' as outlined to date
Your Qs re what to do next are not for us to answer. If you withdraw now then you may, if you wish, apply at a later date with form UN4 to cancel the Unilateral Notice but the beneficiary may object and then ............
The key appears to be that they have an interest which can and has been protected on the register. Removal then relies on that interest having come to an end or not being enforceable. That appears to be where the issues now lie
Any form of Notice will affect a purchase as a buyer will insist that the interest is resolved in some way and the Notice removed before they will complete
If you are reliant on the sale monies to be able to release the option then that's something to discuss with the conveyancer/beneficiaries to ascertain how the purchase might complete and the monies used to pay off the beneficiary - again not for HMLR to advise on I'm afraid