When Mary ascended the throne most wanted to return to Henry's version of Catholicism.
IIRC the English Reformation only got more radical under the reign of her half brother Edward. Henry VIII seemed not to have been that radical on those matters.
And compared to her cousin Charles V, who burned 20,000 plus in the Netherlands, Mary looks like a saint.
You can't compare those rulers because they reigned in such different eras and had such different goals.
I think that 33 years would definitely be enough time to restore Catholicism or at the very least to see the position of the Protestants almost completely destroyed. If that's the case thenI bet you Elizabeth would convert to Catholicism, at least publicly. Elizabeth was a realist above all else. She saw that Catholicism was not the popular religion and to support it would heavily weaken her position. The same would be said if Catholicism was the popular Religion. It Catholicism was the popular religion then Elizabeth would embrace the old Religion, no doubt with minor reforms to make the Church more moderate.
The interesting part about Mary living longer is how. Does she never develop cancer? If so then maybe she'll have a child, who would be in his/her late 20s to early 30s when they inherit the throne. Hell without English aid Scotland might remain Catholic. If so then if Mary I remains childless then we might see Mary Queen of Scots become the heiress to England, especially if Francis II dies in schedule.
20000? Are you sure, I've never seen that number. Maybe for the entire Habsburg (Spain, Netherlands, Italian possessions, the Colonies etc.) Empire. To put it into perspective the Infamous Council of Troubles (AKA Council of Blood), instituted by the duke of Alva, 'only' executed a bit less than 1100, but had exiled 11000.
In fact I've seen estimates of between 4000 and 8000 being persecuted for their religious beliefs in the XVII Netherlands, but those who actually ended up being executed were far less.
Some sources from a part of this region showed, that from the 1473* persecuted persons, 'only' 169* ended up being executed.
So 20000 executed only for the XVII Netherlands seems very exaggerated.
(*= source: W. Blockmans (2000), Karel V: keizer van een wereldrijk 1500 - 1558 (translation: Charles V: emperor of a global empire 1500 - 1558))
In a Catholic England, Elisabeth might indeed, think that the throne is worth a mass.
Even a somewhat different Mary might instead of directly wanting to restore the union with Rome want to make the Church in England less radical again too, which would be more like Henry's Church.
Still it is understandable, that Mary with her family background fled in her and her mother's religion. This doesn't mean, that the means she used to reach her goal was the right one.