I think who her cousin the King of Scotland will be the most likely husband for a Mary who become queen at such young age. For different reasons neither the already made betrothal with the French Dauphin or a wedding with Emperor Charles who Catherine desire so much are good choices for a Queen Regnant of England and both will have powerful court faction (in the same Regency Council also) against them. A Scotland match (and with the man after her in the succession line) can be an acceptable compromise for both factions and keep in mind who the French-Scottish alliance was mostly an anti-England alliance. I do not think who the Scottish regent can refuse an union of the crowns (with James who is both the male and the older of the new royal couple) only for the sake of the old alliance. The Scotland only few years before lose a war and his King against England and a young Queen with the whole England as dowry would be surely a better choice over a French princess.
As you said Katherine was fiercely against the French match and I do not think who anyone in England would like being ruled by the French, specially not by the future King of France and the Imperial match is unlikely because a)Mary is too much young for Charles who is an adult and need heirs fast and cannot waiting years for his bride b)Charles have the Spain problem because he is born and raised in the Netherlands and almost never or never stay in Spain before starting to rule it (and is for that who is younger brother, Ferdinand or Fernando who was born and raised in Spain at their grandfather's court, was quickly sent in Germany) and a Portuguese's bride will be much better accepted from the Cortes
As you said Katherine was fiercely against the French match and I do not think who anyone in England would like being ruled by the French, specially not by the future King of France and the Imperial match is unlikely because a)Mary is too much young for Charles who is an adult and need heirs fast and cannot waiting years for his bride b)Charles have the Spain problem because he is born and raised in the Netherlands and almost never or never stay in Spain before starting to rule it (and is for that who is younger brother, Ferdinand or Fernando who was born and raised in Spain at their grandfather's court, was quickly sent in Germany) and a Portuguese's bride will be much better accepted from the Cortes