I wouldn't be surprised if Pippa decides to adopt the title of Lady Glen Affric after she's married. She'd enjoy that, I think.
I would think that would get her laughed out of the aristo circles she wants to run around in.
Yes because it is a title that comes with the estate not an inherit title. So whoever owns the estate has the title. And she can't use Lady as long as her husband doesn't own it. Her title would be something like "Mrs Matthews of Glen Affric the younger" because her father-in-law is still alive and still owns the estate.
Have just done a bit of googling about lairds, including the web page of the Lord Lyon. "The Court of the Lord Lyon is the heraldic authority for Scotland and deals with all matters relating to Scottish Heraldry and Coats of Arms and maintains the Scottish Public Registers of Arms and Genealogies."
According to that authority, the term "laird" is a description rather than a title. To have any claim to distinction, the owner of Glen Affric would have to have been granted a coat of arms by the Lord Lyon, which signifies nobility. Even then, it would not give him the status of a peer. As far as I can tell, Matthews has not been granted arms. I suspect he would not have even applied for them. From the little I know of him he seems a reasonable sort of person and must have some sense to have achieved the success he has achieved. Since he has no real connection to the property, merely having bought it and developed it for commercial purposes, and does not identify as living there, and does not have any particular association with the region or the people living there, he probably would prefer to preserve his dignity and not to give the powers that be the opportunity to laugh him and his bank account out of town.
I think James and Pippa would be well advised to downplay this connection unless they eventually decide to live there permanently and raise their children there and forge a connection to the land and the local people. I hope James has more sense than to encourage any Middleton zeal on Pippa's part to claim entitlement for them to be known as James Matthews, Younger of Glen Affric and Mrs. Matthews, Younger, which is all they could be known until he inherits the property, if indeed he does inherit it.