Thailand has very clear laws and very clear rules of intestacy. That said, ultimate power rests with a court - which is ultimately responsible for granting Probate.
Wills and signatures DO matter, and will always matter.
Anyone would be foolish to doubt the legal validity of leases and usufructs. My partner could sell 'our' land even with the existence of the usufruct in my favour....that is little different to someone in the west selling a house with a sitting tenant. Nobody in their right mind would buy 'our' land subject to my rights of occupation and to the 'fruits of the land'.
(My partner calls the land 'ours' - I don't, it is hers).