I agree that there were principal lines of Caldwell's other than the one that terminated with the heiress who married Gilchrist Mure.
My research points to the fact that the heiress was the descendent of the Caldwell-Chancellor and that "was the principal line" The senior family of Caldwell's from the Lochwinnoch area then became the Caldwell's of Little Caldwell. Caldwell of that Ilk. It is reasonable to presume that this family was the family associated with the commonly known crest.
The head of theis family was involved in the "Chitterflat Incident" - an offshoot of the general Pentland Hills uprising of 1666. He and many other lairds were forfeit and attainted. He and the then head of the Mure family who was also at Chitterflat fled overseas, I believe that the Caldwell went to the Americas and the Mure to Holland. Later the Mure family were restored to their estates largely due to the efforts of the then Mrs Mure and her daughters. The Caldwell never returned.
There were "hundreds" of Caldwell families - even in antiquity. The chances of any current Caldwell's being descended from the main lines is very remote.
I think that you will find that "William of Stratton" was no lord, but possibly a landholder. I also think you will find that it is not "Stratton" but "Straiton" which is a village in Ayrshire and merely indicated his place of birth.
He was the progenator of the Irish family of note. I think that this family descended originally from the Ayrshire branch of the family and possibly only very vaguely connected to the Renfrew family that everyone seems to be trying to link themselves to.
The Irish branch were originally traders and went into bat stenuously to grab Irish Catholic land when it was being handed out after the "Revolution". They were sympathetic to William of Orange and took part in the defence of Enniskillen.
That they became wealthy and important had more to do with industry and opportunism in Ireland rather than any aristocratic forebears or formner wealth in Scotland. (I have read the book).
When they finally matriculated their own coat of arms there were allusions to the original Scottish Coat of Arms which might indicate that they at least thought they were descended from the original line.