Summary:
The meaning of אי כרב אי כשמואל is whether the צדי הרחבה are considered a רה"י or a רה"ר, or (according to רש"י) whether מחזרת is קרן or שן. However in any event ר' יוסי cannot agree with שמואל, since ר"י is מחייב שן ברה"ר.
But if they fell, they are exempt; it is evident he maintains, etc. - הא נפלו פטורין אלמא קסבר כולי
Summary:
It is תחלתו בפשיעה וסופו באונס either because the vessel is in a place where it can be broken either by the falling or jumping down of the dog, or because it is a long vessel, where the closer part will break by falling and the further part will be broken by jumping.