“Secure” as a 100% guarantee? Of course, not. But in OTL mission of Monluc (with the promises I listed) was successful in Poland (Lithuania was seemingly less happy but followed the suit) so it is realistic to assume that it would be successful with Francis on the throne.
However, if is an open question if in this TL anybody would care enough to give the OTL promises on behalf of Charles or Henry.
Catherine de Medici here will have exactly ZERO political power and the butterflies on French political situation, both internal and external, are really huge.I definitely agree that Charles is likely to be preferred over Henry. This is precisely what I said there:
It is only if Charles still dies of tuberculosis as in OTL that Henry may be elected after him.
In OTL, the idea to make Henry king of Poland came from Sokollu Mehmed Pasha, the Ottoman Grand Vizier. In 1569, when Sigismund II was still alive but expected to die childless, Sokollu Mehmed Pasha received the French ambassador, asked him some questions about Henry and suggested to make him Sigismund's successor.
The purpose was to have an alliance with Poland against the Habsburgs and the Russians. Catherine liked the idea and Charles agreed. Then, they sent Jean de Monluc to Poland.
This is the main reason Henry was elected. Charles wanted so much to get rid of him that he cried when they separated.
As for Catherine, she was not in power in OTL either. Charles was. Of course she had influence over Charles but she had influence over Francis too. And, though it is true that Henry was her favorite, she actually wanted a crown (either as regnant either as consort) for all of her children.
Catherine would, obviously.
I guess we can't know what Francis would think about it but Catherine is likely to convince him if he is reluctant.
In OTL, Marie de Guise ruled Scotland as regent. If Francis and Mary manage to reconquer Scotland, my guess is that one of Mary's uncles would become the new regent (or viceroy if you prefer).
Considering all the trouble with the Lords of the Congregation, there is zero chance that Francis and Mary may let some "local noble assembly" govern without someone trustworthy above them.