G M Paul - the initial enquiry has been flagged as an 'amend' so in essence a possible correction. I can see that you have followed up with a number of additional emails/information as well. The complexities involved are likely to mean a significant amount of investigation is going to be required, inc paper records, so it will unfortunately take time. Hopefully though the wait for a n answer won't be too much longer
As to your question now posed here it doesn't read like they would be claiming ownership through adverse possession as you refer to the original owner being the legal owner so presumably have proof of title. The beneficiary has then inherited the land in effect so if that's the case you are dealing with an application to register the land based on a transfer by the deceased's executors to the said beneficiary.
It's possible you have something to confirm the deceased's ownership but other deeds have been 'lost' - if so then any application to now register would rely on a statement of truth re what's happened re the said deeds etc -
see PG 2
However if there are no deeds to confirm legal ownership then a claim is the only way forward and such a claim would be based on the facts as known. Those maybe limited for the reasons you mention but the key start point is always the legal requirements and have those been met. The registration ones would come next so it is important to eek legal advice to understand what evidence you have/is required in law to prove ownership. If the legal requirements are met then an actual application to register the claim can be made