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I assume they did not do anything with the cooling system?
Did you get the mechanic to put the information he gave you in writing?
If they did nothing with that hose, you will have a hard time making them responsible. Even though you have documentation on what they did wrong, those things do not seem to have anything to do with the failure of the clamp or the hose. Add that to the fact that you went 400 miles after the service will make it hard for you to win this one.
Sorry to give you news you might not want to hear but I just want to give you an honest opinion from an outside observer.
non-synthetic oil will not hurt. Overfilling signifigantly could cause it to lose oil and smoke. But it really depends on how overfilled it is.
I don't know if it is a coincidence if you went 400 miles and there was already a leak which was not fixed.