I don't claim to be a Buchanan "expert", but the question as you pose it is literally impossible to answer. The Clan Buchanan has a small fraction of Morrison's who came from Perthshire centuries ago who are members of our Clan. I just met one recently who did significant research to discover his roots reliably and came to that conclusion.
I don't mean to be disrespectful but throwing out names and places and asking anyone to explain "who went where and how" is impossible.
If this is a question about your family then the only way to answer the question is through genealogical research which is painstaking but certainly rewarding. Modern DNA testing is also beginning to make some inroads on questions of clan affiliation.
A more general answer is that there are many, many surnames associated with multiple Clans according to the most commonly used references. Which particular clan is the right one for any one individual is a very difficult question to answer without research or very specific DNA testing.
Many Buchanans went over to Ulster during the Plantation experiment and many came back to Scotland and many more went to North America or Australia. Because Buchanan was originally a place name in Scotland, the name does not originate anywhere else unlike some names which are descriptive or patrilineal and originated in a number of places i.e. Wales, England, Ireland etc. So literally all Buchanans can eventually trace their name back to Scotland originally.
Sorry I can't be of more help than that.
President, Clan Buchanan Society International
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