The French match is way too much prestigious for renouncing to it, and Edward can very well leave the religious question unresolved, hoping to persuade Elisabeth to convert after the wedding. If the French wedding fail over the religious question and Marie Stuart became available, then annexing Scotland is too much important for caring too much for her religion, so is likely who in both cases Edward will try to persuade his wife to convert while raising their children as protestants.If he is to marry a Catholic, then either:
It must be agreed that she will convert to Protestantism before the wedding. I can't see that happening with Elisabeth de Valois.
Or:
It must be agreed that she may continue to practice Catholicism in private - with a personal confessor, etc. This was the condition of Charles I's marriage to Henriette Marie of France, who brought twelve Oratorian priests in her personal entourage. I can't see Edward agreeing to anything like that.
So IMO he won't marry Elisabeth.
OTOH, he might marry MQoS without requiring explicit conversion in advance, if he can restrict her household to Protestants and prevent her from practicing Catholicism. This might be possible if he seizes control of Scotland (say with the support of the Scots Protestant faction). MQoS might be bullied that far, while Edward may believe (as suggested) that he can persuade her over time.
WRT MQoS's first husband Francis II: AIUI, his health problems were congenital, thus would not be affected by Edward's survival.