It's a very good question why MD never became Countess of Angus - she was after all the Earl's only daughter. I think that Scottish succession was still abit vague on when daughters could inherit. After all, the issue had only really been settled in England by the Wars of the Roses. After 1603 I'm sure that in Scotland the only daughter would have succeeded automatically, but before that I suspect it happened only if there wasn't a close relative who was a better bet.
In this case the Earldom and the lands went to Archibald's nephew (who died a year later, leaving a two year old son, the eighth earl). I'm sure everyone went along with this, as noone wanted to see the Earl of Lennox made more powerful, especially not Archibald, who had been an enemy of the Lennoxes, and as a Douglas would have hated to see a Stewart get the family lands.
It seems unfair on his daughter, but they were certainly far from being close - I don't know if they met at all after the early 1520s! What a family..... I agree with PL that Margaret never met James V or Mary Queen of Scots either. She journeyed north in the 1570s and got pretty close to where Mary was staying at the time, which was enough to earn her yet another spell in the Tower of London!