Right, I’m less tired now, so here we go:
Why are you ‘winnowing out’ other clauses?
The main clause there is ‘he had arrived at his proposition.’ There are several subordinate clauses in that sentence, of which ‘for it isn’t’ is one subordinate clause.
‘He had arrived’ (…) is not part of that subclause.
There the main clause is ‘It is easy to give an account of why continuity of brain and memory should be evidence of it.’ (Within that, ‘an account of why continuity of brain and memory should be evidence of it’ is a very long noun phrase).
I can’t find the post where you posted that novel sentence in full.
‘My dog has fleas’ would be a main clause, but adding ‘for’ turns it into a subordinate clause.
I don’t understand why you’re winnowing the bits out that you are. You don’t seem to have any understanding at all of what a main clause is.
A sentence fragment is a sentence that has no main clause and is not one of the exceptional clause types (such as orders, certain question forms and exclamations); there is no other definition. It’s fine to use them sometimes, but you should be aware that you’re doing so.
BTW, you asked for a cite saying that a subordinate clause is not a grammatically complete sentence: you were given three such cites earlier in the thread.