However, not everyone was so delighted. Mary the Spaniard, the elder sister of Edward, had been waiting now for over twenty years to clear her own name and restore the Catholic faith in England against the Protestant heretics; now her chances appeared to be retreating again. Once more, she was plunged into despair. Her life in England was now becoming intolerable. Over the autumn of 1553, she entered into correspondence with her cousin, the Holy Roman Emperor Charles V, who agreed to begin another secret expedition to extradite her from England.
Mary was heir to the throne by Henry VIII's Third Succession Act, which at that time was the law of the land. Though Mary had stormy relations with her younger brother, up to and until Edward VI had a living child Mary was the legal and popular successor to Edward VI. She rather easily overthrew Northampton's plot to place Jane Grey on the throne, and that popular support came from both Catholics and Tudor legitimists. I don't see why she would throw that support away.
Neither Northumberland nor Parker could possibly have known that their actions would lead to what became one of the most enduring alliances of the period; and one that would eventually spell the doom of the Hapsburgs and their vast dominions. Indeed, for now, Parker complained bitterly of being forced to set out across freezing and stormy seas for Muscovy.
Ivan IV did not have direct access to the Baltic Sea. He also was involved in fighting the Swedes, Poles, Lithuanians, and Teutonic Knights, all powers bordering the Baltic Sea. I don't see what possible use an alliance with Russia would have for England. England has little strategic use for Russia, and what little it has is based on the Muscovy Company, which was already forming regardless of marriage negotiations.
If Elizabeth is to be married, it would be within the Western European alliance system, a system that Russia is not yet involved with on any level.
Princess Elizabeth was also rather unconvinced by the plan. In London, Christmas quickly descended into a violent struggle at court between herself and Northumberland, who was attempting to persuade the King to bastardize his sister in favour of his cousin, Northumberland’s daughter-in-law, Jane Grey. In this, Elizabeth won out. Her brother flew into a rage with Northumberland, and seriously threatened to remove Jane totally from the line of succession, let alone promote her. Chastened, the minister retreated. It was the first hint of the Edward that was to emerge; a man devoted to his family and their well being, and, like his father, only too willing to cut down overly successful ministers.
If he is like his father, then a minister demanding things like this of him would cost that minister his head. This seems an excellent time for Edward VI to take real power in England, and have Northumberland executed. The marriage of Northampton's son to Jane Grey is a direct threat to Edward's line, since that makes Northampton loyal to a possible pretender's line. If Edward VI is a Tudor then he will execute Northumberland, and probably Northampton's son and Jane Grey.
1557- Edward marries 12 year old Elisabeth of Valois.
If Edward is eager to secure his line on the throne, then would he turn to another foreign bride or possibly a domestic bride of childbearing age?
1558- Archbishop Cranmer is assassinated by an extremist Marian supporter. Edward begins major purges of Catholics, but refuses advice by the new Archbishop, Edmund Grindal, to burn them at the stake. A steady trickle of Catholics begins to flee England to the court of the exiled Queen Mary.
Henry VIII beheaded open Catholics and burned evangelical Protestants. Though burning heretics was brutal, it was also the common policy, and Edward VI would probably engage in it, champion of Protestantism that he is.
So if Edward VI is a Tudor then he will get rid of overmighty subjects and burn heretics. Northampton is an overmighty subject, and the assassins of Cranmer are traitors and heretics.