WI: Mary, Queen of Scots remains Queen

Let us assume that Mary, Queen of Scots is not deposed from her throne in 1567. Her first marriage to the French Dauphin still ends in his death and without any children, and she returns to Scotland. Assuming her marriage to Lord Darnley is somewhat stabilized and perhaps he does not antagonize so many factions (thus not ending up murdered) is it possible for her maintain her throne?

The biggest question concerns England; if Mary remains in control in Scotland through whatever situation, she does not end up imprisoned and later executed by Elizabeth. If she still lives into the 1590s, what steps might the English Privy Council take to secure the succession, as Elizabeth was hesitant to name a successor. Obviously Mary or even James VI are out of the question. Does one of the other viable heirs in England receive the crown?

The Spanish Armada is also an interesting question--I imagine it would no doubt still occur, as England will probably still continue to support the Dutch Rebels. Indeed, if Mary remains on the throne I can also see England supporting the Scottish Protestants, perhaps even clashing with Scotland militarily.
If Mary remains Queen of Scots (and that's hard to manage because of her religion and character) she becomes even less politically powerful over time. She is regarded as a foreigner and a heretic by the majority of her council and subjects...however the public hatred for her is avoided if she remains infatuated with Darnley. The Scots conveniently forget the mobs demanding that she be burned or worse after her marriage to Bothwell and the allegations about the death of Darnley.

He needs a major character change (less vain, less demanding etc) - however you might achieve that if you allow Elizabeth I to let his mother Margaret Countess of Lennox to travel to Scotland - she appears to have been the only one he was scared of!

The English issue is different - England remains cordial to Scotland - Mary continues to nag Elizabeth grant her her rights to succeed her and Elizabeth continues to prevaricate.

The Scots Council are going to continue to insist that Mary's heir is brought up as a good protestant so if Mary predeceases Elizabeth then union goes ahead. Elizabeth was in private a stickler for the rights of sovereigns her biggest issue with Mary was that she probably believed she was the rightful legal and moral heir to Elizabeth so if Mary outlives Elizabeth it might well force the council to name her son as heir instead. In those circumstances Mary approaching sixty might be forced to live with it.
The alternates to Mary's line (and we can assume she would have had more children) wouldn't have appealed to Elizabeth (or many on her council)
The declared illegitimate children of Catherine Grey (heirs at law under Henry VIII's will if you accept the validity of her marriage), the Earl of Derby and his daughter's (next in line under Henry's will) or Darnley's younger brother's only child Arbella Stuart.