There is a dispute whether the סדר נזיקין is one מסכת or many.
There need be justification why any מסכת follows its predecessor.
Our מסכת continues where we left off in the end of בבא מציעא. There the משניות were discussing laws of division among joint property owners. This theme is carried over in our משנה.
בגויל זה נותן ג' טפחים כולי – By unfinished masonry, this one provides three טפחים of his property, etc.
The משנה states that בגויל זה נותן ג for if it would have stated simply בגויל ו we may have mistakenly assumed that each partner provides ו' טפחים.
The measurements of the משנה are the upward limits. If the מנהג is to make it narrower we follow the מנהג (just as we see that we follow the מנהג of הוצא ודפנא).
When the גמרא states that הכל כמנהג המדינה comes to include הוצא ודפנא, the intent is to provide us (also) with a lowest limit to the sturdiness of the wall.
לפיכך אם נפל הכותל וכולי – Therefore if the wall collapsed, etc.
The משנה teaches us initially that (under certain circumstances) either partner can coerce his neighbor to share in building a dividing wall. The משנה then concludes: 'לפיכך' – therefore, since either partner can coerce the other to build, we can safely assume that they both built it together. That is why in case the wall collapsed, they both share equally in the material and space of the original wall. We may infer from the משנה that without the 'לפיכך'; i.e. when one partner cannot coerce the other to build a wall, then the דין is not necessarily that they divide equally.
סברוה מאי מחיצה גודא – They assumed; what does the word מחיצה in the משנה mean? – A wall!
The גמרא uses the term 'סברוה' for the first לשון to indicate that we do not follow this opinion להלכה. Another approach is that 'סברוה' can refer to an acceptable opinion that is not subsequently refuted or retracted.
In the case of מחיצת הכרם שנפרצה it is the obligation of the בעל הכרם to rebuild the wall. The בעל הכרם is considered the מזיק, since he is not permitted to plant within ד' אמות of the boundary line, unless there is a dividing wall. Nonetheless, unless the בעל הכרם is explicitly warned to rebuild the wall, he will not be חייב for the איסור כלאים, since he does not realize that it is his obligation.