Catherine Howard's name is alternately spelled as both Katherine and Katheryne. her birthdate has been alternately cited as both 1521 or 1525, which means she was either fifteen or nineteen, not seventeen, when she married Henry.
At the time of the episode, Lady Elizabeth (later Queen Elizabeth I) would have been just approaching her 7th birthday. Laoise Murray, who plays her, was almost double that age at the time of filming.
Henry VIII married Catherine Howard on the 28th of July 1540 - the very same day Thomas Cromwell was executed.
Edward Seymour says to his wife that Henry Howard has royal blood making it seem that Edward himself doesn't. In fact the two were second cousins and both had Plantagenet blood.