Well the lords of Galloway were already vassals of the Kings of Scots so if the dynasty survives the likelyhood is that they would integrate further into the mainstream of the Scottish aristocracy. There would be significant butterflies however, given that the Balliols would not gain a presence upon the Scottish stage or any major interests north of the border. Also, any legitimate male-line descendants of Alan would be in line for the throne should the main Canmore line die out - Alan married, if I remember rightly, a daughter of David, Earl of Huntingdon, who was the brother of Malcolm IV and William 'the Lion', and the Balliols inherited a claim to the Crown through a marriage to one of his daughters. So assuming that the survival of the Galloway line doesn't butterfly away the death of Alexander III without male heirs, then Alan's legitimate heir would be next in line to the throne.
Of course if Thomas manages to succeed then he wouldn't be able to ascend to the throne, as he doesn't have royal blood flowing through his veins.
