I think I can answer your questions regarding Airth castle. I am currently researching the Fyfe line, which is very closely connected to the Airth line, and in volume 1 of the Fyfe geneology book, there are several pages regarding the castle. I will tell you in part what it says;
"The first known date connected with the site on which the present castle stands is 1248 when Adam de Erth held considerable lands in Sterlingshire. It is presumed that there was a castle on the site during this time, the oldest part of the present castle dating from between 1550 and 1600. The lands and castle continued in the hands of the Airth family until about 1450 when Agnes, one of the three daughters and co-heiresses of William Airth married Edward Bruce, 2nd son of Sir Robert Bruce of Clackmannan, bringing with her the lands and Barony of Airth. The Bruce family continued in occupation until about 1648 when the castle came into the hands of the Elphinstone family through the marriage of Jane Bruce and Richard Elphinstone. Charles Elphinstone, son of the above couple, was killed in a duel, thus leaving no heir to succeed and the estate was acquired by purchase in 1717 by Judge Graham, whose descendants still own it."
I hope this answers your questions about the castle. We are planning a trip to Scotland in the next few years, and hope to stay in the castle. If you have any further questions, I have a bit more about the castle and the Airth name. Windy