I know that it is very convenient to the plot to have death tax problems and have Lady Mary inherit Matthew's half interest in the Abbey, BUT.
If it were so easy for Robert to deed away one-half interest in his estate, wouldn't that threaten the idea behind entailed estates? That is, the idea is to keep the estate in one male inheritor's hands. Other than losing his estate to creditors or bankruptcy, Robert might not have been able to deed his interest to Matthew in the first place. Now, of course, Matthew was next in succession, and Robert could have ceded his all his rights to Matthew. If Robert were entitled to deed away a part of his property rights, then wouldn't that defeat the object of the inheritance laws, which were primarily for the purpose of keeping these noble estates directly loyal to the royal family? Couldn't he just deed his interest in the property to his wife or three daughters?
Secondly, all the same reasons should apply to Matthew's ability to grant his property rights in the Abbey to his widow. After all, if Robert's father had been able to deed his estate to Violet, she would not be called the "dowager." She would be the holder of title to Downton Abbey.
I realize the turns and twists of the plot are more important, but can anyone explain how the plot fits in with English law that then existed?