I would like to explore other possible way to have Elizabeth married: make Mary I living longer. IOTL Philip was affraid that if Mary would die in childbirth the throne would go to Mary of Scotland, who was then betrothed to the Dauphin of France. So he suggested marrying Elizabeth to the Duke of Savoy, a Catholic ally, in order to keep England pro-Spanish. But it seems that when became clear that Mary wouldn't conceive a child and her health wasn't good he instead started to think about marrying her sister himself. Of course, once Elizabeth became queen she refused it.
However, suppose that Mary lives longer and really becomes pregnant ITTL, but as her mother (Catherine) she has several children with a very short life or are stillborn. So, while the succession of Mary is still dubious the queen is seen as healthy, and so Philip can't even think about remarrying with Elizabeth. So he decides to push her into a forced marriage (with the Duke of Savoy or other Catholic ally). So Mary dies later than IOTL, but still childless, and Elizabeth becomes queen, but she is married with a Catholic foreigner. Is it a likely scenario?