I'm finding this thread rather confusing.
Both the original question and the restated one refer to Heraldic Visitations and Coats of Arms, but somehow we are now discussing land ownership. The two subjects are related but not the same.
To pick up on supermoussi's example : "Could these unlisted Marmadukes be descendants of Sir Walter's youngest sons, Richard & Henry or does the fact that Sir (Robert) Marmaduke's land went to a female descendant mean that there weren't any direct male descendants to inherit the land?"
Correct that there are no direct male descendants.Sir Robert Marmaduke had brothers but not sons.
His land has gone to his daughter who, as heiress, carries it with her to the Montagues. Richard and Henry have no claim to it.
With regard to arms, Sir Walter's original Arms are carried through to his grand-daughter and, after her marriage, may appear "impaled" with her husband's. Later Montagues may "quarter" the two arms to recognize the lineage.
Richard and Henry, being the sons of an "armiger" have the right each to their own arms. Often these will be Sir Walter's arms differenced in some way e.g.: by changing a field or a colour or adding a charge . ( Note that this is not the same as Cadency, which is a separate issue.) Alternatively, they may choose to adopt (assume) new arms for themselves.
By the time of the Visitations, many generations later, the descendants of Richard and Henry may have slipped so far down the social tree as to no longer be considered of appropriate status, and may well not be aware of their family background.
There is also the possibility of illegitimate descent, where the father has recognised the child as his and given him his surname, but no legal status inside the family.
Maec