"Margaret must have died sometime between the birth of her last child George in 1553 and the remarriage of her husband to Anne Horseman in 1560."
Where does the birth date for George come from? He was not mentioned in his paternal grandmother's 1552 will. AFAIK, this is the only data point for him, ie b After 1552.
BTW Robert KNAPP did NOT marry Ann HORSEMAN in 1560. She was the widow of Robert HORSEMAN.
This extrapolation from known to guesswork renders this whole thing untrustworthy.