Edward I originally wanted Margaret's grandfather and guardian King Eric of Norway to send her direct to him in England in 1290 so he could use her as a pawn and dictate to her Scots regents as to when she would marry his son. In the event Eric turned him down and let the Scots embassy sent to collect her take her home but she died en route at Kirkwall, Orkney - the delay in her departure from Norway from 1289 into 1290 may indicate his anxiety at her frailty (making her early death likely in any case) but it may just have been due to her young age (born 1283). I've speculated in one of my books that if she was sent from Bergen to England (Hull or Ipswich?) in 1290 as Edward wanted, the more Southerly route for her ship might have spared her from getting a chill on board an unheated ship off the Orkneys or Shetlands and dying of pneumonia - but this is only a guess as to what happened.
If she had ended up in England, a titular 'betrothal' ceremony to Edward's son Edward of Caernarfon (a year her junior) could have occurred quickly - and was seen in Church law as being as legal as a full marriage. But the normal age for a full marriage and the participants living together would have been around 14, ie in this scenario probably around 1297-8. The young couple would probably have stayed under the aggressive and thoroughly 'controlling' Edward I's supervision until they had at least one boy, who would be Edward I's ultimate heir so he could keep this child in England and permit his son and daughter-in-law to live in Scotland.
More likely he would want them to have at least two children before he let either the young couple or one of their children go to Scotland as ruler - and then he would insist on attempting to get English nobles on the Scots monarch's council to 'assist' the Scots elite in staying loyal to him. His father Henry III had insisted on having Englishmen on the regency council for the young Alexander III (Margaret's father) as an under-age new monarch in the 1250s, to represent his interests as A's father-in-law, ally, and supposed 'overlord' - this manoevure had recent precedents. The Scots denied that Edward as King of England was the overlord of the King of Scots and Alexander had refused to do homage to Edward - but Margaret , in Edward's control as an under-age female and as his legal ward (he was her only surviving adult male relative) after 1290 would have had to give in. Any Scots noble or non-noble revolt against this arrangement would have been put down by English troops, as the revolts after 1297 were in real life; and both Balliol (a County Durham baron) and the Bruce line ('our' Robert Bruce's father, then him after 1304) had lands in England which Edward could confiscate so they would probably play a 'waiting game' as Bruce and Comyn did in real life in the early 1300s.
Edward II was notoriously at odds with his father's manner of kingship and his elite of senior noble advisers, and to the horror of conservative nobles and the mystification of chroniclers preferred digging, rowing, attending rural fairs, and other 'non-noble' pursuits to the accustomed world of courtly tournaments and warfare, quite apart from his involvement with the sharp-tongued and acquisitive Gascon 'outsider' Gaveston. (He was however keen on horses and was capable of fighting when needed, if only under pressure.) Given his hostility to his father's milieu and the latter's at times violent bullying - Edward I once pulled his son's hair out and on another occasion snatched a coronet off his daughter's head and threw it in a fire, so the portrayal of him in 'Braveheart' is not that inaccurate - both he and Margaret would probably have 'bonded' in mutual loathing of the elderly King. Edward I proposed when he was attempting to get Margaret from King Eric that he would keep her in England under his 'care' until she had an heir, but given his suspicious and devious character he would quite likely have kept her in England for the rest of his life and if she tried to run away sent knights after her to bring her back. if she and her husband E II were allowed to govern in Scotland leaving a child in London, this would be after c1300 and probably 'controlled' by a partly English council - possibly as per the real life governing council of 1300-07,dominated by a reliable English noble like Edward's cousin Aymer de Valence (earl of Pembroke) or the earl of Surrey.
Balliol, a timid character forced by his nobles to fight Edward in 1296 and once he was deposed content to stay on his French estates not come back to Scotland to help Wallace and Murray, would have stayed out of any revolt; Robert Bruce junior is more likely to have risked it, especially if Margaret was back in Scotland (after Edward I died?) and defying the new regime in London and she rallied her people. The same sort of revolt would occur if she was still in London and a child of hers was nominal sovereign in Edinburgh, governed by a council named by Edward I? If Margaret had a son (Alexander IV?) to be nominal ruler of Scotland and he was kidnapped/ freed by the rebels to be their nominal leader, he could be married off to Bruce's daughter Marjorie so Bruce was the regent and strongman.
If Edward II and Margaret had a better relationship than E did with the strong-willed, extravagant, ruthless Isabella (who has been blackened by later writers for her behaviour in 1327-30 but was arguably reacting to humiliation by the Despensers and her fickle husband and may not even have killed the latter)
, once he was King the two might come to an amicable agreement for Margaret to live part-time in Scotland. Her nobles would have insisted that the crowns were kept divided if they had two children, with one inheriting England and one Scotland.The dynamics of combining the realms were too difficult to work long-term in that era, and if Margaret went along with such a plan she would face a revolt. Nobles would use her heir against her, as the young James IV was used against James III in OTL (1488) or James VI was used against Mary (1567). If by some twist of events Edward I allowed his son to govern Scotland as co-ruler with Margaret in his lifetime, he would have to return to London on his accession there in 1307. But if he did end up with a 'favourite' while governing in Scotland and alienating the Scots elite as he alienated the English one in OTL, then he would have gone the same way as he did in 1327 - and probably quicker as the Scots king had fewer resources to hold out once most of his nobles defected. Witness the mass-murder of James III's unpopular 'low-born' ministers by his nobles in the 1482 revolt and his subsequent removal in 1488.
