Cecily Plantagenet had only 2 legitimate husbands.
Although you are correct that betrothals in 15th century were legal contracts, whereby the female was entitled to the titles and privileges of her future spouse, I don't think I would classify a betrothal as an actual marriage. Legally, the two parties were joined together, but without a wedding and consummation, neither party was entitled to the inheritance or property of the other, unless it was specifically addressed in the contract.
She was first betrothed to the future James IV of Scotland in 1474, but due to military conflicts between England and Scotland after this date, the agreement was nullified. Officially, however, she would have been known as a Princess of Scotland and the Duchess of Rothesay from the date of her betrothal. Once that agreement was ended, she probably ceased to use those titles.
In 1482, a second attempt at a Scots match was made, when Cecily was betrothed to Alexander Stewart, Duke of Albany. He was the brother of James III of Scotland, and the uncle of previous fiance, but he was killed on 7 Aug 1485, before the marriage took place. This time, from the date of her betrothal, Cecily would have been known as the Duchess of Albany.
Edward IV had died in 1483, which changed the marital landscape for his daughters considerably. Cecily would have been the wife of Alexander Stewart had he lived to marry her, but the usurpation of the English crown by Richard III, changed both the politics and her future.
Of course, Richard's reign was short-lived, and when Henry VII acceded and married Cecily's older sister, he created an additional alliance by marrying Cecily to John de Welles, 1st Viscount Welles.
John was the half-brother of Margaret Beaufort, and therefore an uncle of Henry VII. They were married in December 1487. So it isn't possible that they had a son named Robert in 1484. At that time, Cecily was officially known as the Duchess of Albany, and was still betrothed to Alexander Stewart.
John and Cecily had 2 daughters, but neither survived. There is no record of them having a son, or at least none I have seen.
John died in 1499, and Cecily married Thomas Kyme sometime after this date. The marriage was performed without the King's permission, and as a result, all of Cecily's estates were forfeit to the Crown and she was banished from court.
Eventually, she did regain most of her estates, and she and Thomas were the parents of 2 children, who unfortunately didn't survive either. She died in 1507, without descendants.
There is no record of Cecily Plantagenet ever being married to Thomas Howard.
Thomas Howard, 3rd Duke of Norfolk, was married to Anne Plantagenet (Anne of York) in 1494. She was Cecily's younger sister, having been born in 1475.
At the time of her marriage, Thomas Howard was still the Earl of Surrey, and she was known as the Countess of Surrey. This couple had 4 children, but the oldest surviving child, Thomas, died at the age of 12 in 1508. It is very likely that Anne herself died in childbirth in 1511.
Thomas Howard then married Elizabeth Stafford, the daughter of the Duke of Buckingham in 1512. She was the mother of his 4 surviving children, and Thomas became the Duke of Norfolk in 1524.