There is a not a great deal more that I can add other than it depends on the lease.
The concern of a buyer and a mortgagee is if the responsibility for maintenance of the building as opposed to the flats, which are most often the flat area and everything inside up to an including the plaster, then your freeholder is two people one of whom is awol, and the other less than motivated. What happened in the past is irrelevant it is what the lease says. Now if the lease includes in your demise the walls and structure and services and the other one similar then the landlords obligations are relatively few. However if one or other is in breach and there is no one in place to enforce the covenants then that is another concern...
As to the freehold ownership currently you are buying a lease and that's all the BS are interested in. An absent freeholder can be dealt with exercising the right to manage, .The shorter lease issue can be resolved by application to the Court and LVT, once as explained you have both or she, for a lease extension, have advertised and investigated to the best of your ability and can convince a court. A payment will have to be made representing a lease extension vlaue.
However unless the deeds to the freehold (or the lease or if the FH is owned by a company , in its Articles) require the transfer of the freehold on a sale of a lease, I cannot see a right to force their interest into your hands, it remains the property of lady and Awol. She has to convince the Court to empower to make decisions in his absence as the freeholder.She might be able to convince them to transfer from AWOL + Her to Her + you, but on balance I am not sure a Court will be happy to do so so soon.
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers. More ramblings atleaseholdpropertymanager.blogspot.com