I did mention Glen Garioch way upthread. I don't think the origin of this is Gaelic and have read claims that it is Doric (a language that is basically English with a strong accent and local idiom). I've no idea what it means.
-ich and -aich is going to put non lingusits to sleep, I'm afraid. In Gaelic, there is a rule that if you have i or e (slender vowel) one side of a consonant group, you must have an i or e the other side. Similarly, if you have an a, o and u (broad vowel) one side, you must have an a, o or u on the other side. Often this results in a purely decoradive, silent vowel. Like all rules, though, it does have exceptions. In a'chladaich (genetive form of shore), the a after the d is silent - tempting to say that it is purely to adhere to the broad to broad rule, although it is actually a product of creating the genetive form. It is, though, pronounced as though it were spelt a'chladich.
In terms of whether everyone says "brook laddie" - they might say that in some parts of the world, but I have genuinely never heard it with a Scottish accent. That doesn't mean it is never said in Scotland, just that I have always heard it pronounced as it is spelt - that is, with the same ending as Caperdonich, Glenfiddich, Pittyvaich and Teaninich. And with a similar sound in the middle.