The point of this is that we are imposing what happened in OTL when the council fell apart very quickly and the country turned pretty quickly to Mary.
Edward's device had three motivations - 1) to prevent a Catholic succession 2) to prevent a woman from ruling (his early drafts were clear and would have established male only succession - the throne was to pass in turn to the heirs male of his Protestant female relations - Frances, Jane, Catherine, Mary and Margaret Clifford - the assumed male to be governed by his mother until his 18th - it was amended to name Jane and her heirs male in the final draft) 3) To ensure the continuation of Edward's Protestant reformation.
In OTL
The majority of the notables who supported and agreed to Edward's device in his presence later claimed they'd been bullied to do so - however few protested at the time (they were of course later trying to persuade Mary of their loyalty to her).
There is debate over how much Northumberland influenced the device, but he certainly wished to carry out Edward's desires.
Northumberland made one fatal mistake - he failed to secure Mary's person before Edward's death.
Foreign diplomats in England were convinced Mary was more popular with the populace than Jane (which she was) but they also believed the council-backed Jane would be established as Queen.
The biggest problem facing the establishment of Jane was her succession was in direct contradiction of the law - she was legally only fourth in line (arguably she was third as Frances Brandon appears to have been excluded from Henry's will).
Mary's ultimate victory was a vindication of the law - she was the legal heir (irrespective of views of her illegitimacy) as established by her father's will (which had legal authority under Statute)
An important facet to Mary's popularity (and it applied to Elizabeth too - one of the problems facing Mary through her reign) that unlike Jane they were the daughter's of Henry VIII. Legitimacy aside people viewed them as having an absolute right to succeed over a more distant cousin.
The council got cold feet - several key nobles started to worry once Mary mobilized support in East Anglia - that saw some such as Arundal move to act to save their skin and proclaim Mary and arrest Northumberland.
However, many of them were already uncomfortable because of the legal position (had Edward's device and will been given Parliamentary approval as he planned then things might have been different), and concerns about the undue influence Jane's accession would give Northumberland (although after Somerset's fall he'd been careful not to try and dominate the council and was never Protector etc) and natural fear they might end up on the wrong side.
In TTL
The legal position is clearer and in a manner most would understand - the legitimate above the illegitimate. (In Henry's mind he was legally only married twice - to Jane Seymour and to Catherine Parr) - this isn't just a matter of the morality of setting aside Mary and Elizabeth - both were legally under statute illegitimate.
Edward VI will undoubtedly expect to be succeeded by his legitimate sister - it gives him two things he wants above all - continuing Protestant reform and prevents a return to the Catholicism of the past.
To courtiers in OTL concerned about the legal subversion of the succession in OTL - you have the King and his council acting to maintain the law.
They also are faced with choosing a young girl which allows them to retain authority and power for some time and someone who can be married to best suit their advantage or a woman approaching middle age, with a long-memory of those who have served her ill over the years, a woman who may not be able to produce an heir given her age leading to even further uncertainty.
It will in the end boil down to how Mary behaves - does she try and over set her father's will and motivate enough support to force the council and Northumberland to back down?
That is in my view debatable - Northumberland moved to defend the Channel from Spanish/Imperial support for Mary in otl - and was given assurances from France they wouldn't act in Mary's favour (Mary's accession would not appeal to them at all).
Her domestic appeal - a true daughter of the late King being kept from her rightful throne by a distant cousin backed by corrupt councillors - won't wash in ttl.
Also the succession is not a rushed thing suddenly dreamed up as the King realised he was dying - it has been established for six years - and has been apparent to everyone for that period - no complicated explanations of the new monarch's claim and right have to be given (as they were for Jane), there is no element of shock or surprise to offend people.
An interesting point about Mary - throughout her reign, her conviction God would give her an heir of her body not withstanding, she never moved to set aside her father's will or amend the succession - despite the best efforts of her most loyal advisors and the fact that she had relatives she much preferred, and one of them was nominally Catholic, she left Elizabeth as her heir - so her actions might be different in this tl.
England's soldiers (a disorganised rabble of veterans quite frankly from disparate backgrounds as England had no standing army as such) had just spent years fighting French and Scots Catholics they are not necessarily going to turn en mass to support Mary - many will but a lot won't.
England is probably at heart still majority Catholic (or Henrician Catholic) - Edward's Protestant zeal had prompted fear and anger - Mary will be seen as a beacon for halting that change - and it will generate support for her.
But very few revolts that come from the bottom of society win out - for victory Mary needs certain elements beyond the support of the masses at this period - she needs the country gentry and the higher nobility and she needs speed and a collapsing centre - I am not sure in this scenario she can quite get that.
Assumptions -
If Mary wins (and she may well succeed given a bit of luck) then she also faces a problem - she has subverted the succession for her own gain (whatever her moral rights are) - she will as in OTL reverse her illegitimacy - given a justification after the fact of her actions.
Secondly - she is now facing a reign with a succession crisis - she has deposed her sister and presumably placed her in the custody of devout Catholics but her sister is as stubborn as the rest of the family and has been raised a Protestant in her brother Edward's mold - it may not be easy to turn her into a devout Catholic and what of Elizabeth?
Who is the legal heir?
Mary has subverted things - the throne should pass to her next sister (but she can point out that Elizabeth is illegitimate) so the throne goes to the young girl she has supplanted - a rather awkward conclusion.
As her unpopularity grows (the Spanish marriage, failure to produce an heir, her growing religious zeal etc) rebels now have two heirs to focus their attentions on and one of them is too young to be accused of treason or disposed of - the survival of the two Protestant sisters is probably assured. Elizabeth is hard for Mary to marry off in this scenario - she only removes one threat - and Elizabeth's succession is now not assured to a relative of the Emperor isn't necessarily going to want to bite - a domestic marriage would present too much of a threat to Mary)
On Mary's death - all hell breaks loose - two intelligent women one in her mid teens the other in her twenties - both unmarried - which succeeds?
