Hi Jennywood and welcome to TP
I am afraid your post made me well up with tears, as I am sorry to tell you that this is all too typical. Many on here have had the same experience, although, as you say, it makes no sense....
I can only suggest that you keep a record of your (to be) mother in law's condition, and request a re-referral to the Memory Clinic if needed.
Meanwhile, ask for help from the GP, the local social services, Alzheimer's Society, and whoever is available in your area. Do you have access to a specialist nurse? ( sorry, name of service escapes me).
I look after my mum and have felt really let down by the memory clinic....however, others may have better experiences and more ideas.
Take care of yourselves too. You and your fiance are at the beginning of a wonderful journey together and you must find support for yourselves.
Good luck and all the best
Lindy xx