Summary:
According to אביי the reason that the משנה teaches us the last דין that they should both make a (stone) חזית is to protect people from swindlers.
אם עמד וגדר את הרביעית – If he arose and fenced in the fourth side
Summary:
The גירסא in the משנה is ואם עמד וגדר, not ואם עמד ניקף וגדר. The גמרא in ב"ק that attempts to derive from our משנה that זה נהנה וזה לא חסר is פטור, indeed maintains that ואם עמד refers to the ניקף, and is following the לישנא אחרינא in our גמרא.
Summary:
In our משנה he is obligated to pay דמי קנים בזול (not just a זוז), because there is the אנן סהדי, that were he able to obtain קנים בזול, he would certainly fence in his field with these קנים.
שניה ושלישית לא – for the second and third wall he does not pay.
Summary:
תוספות and רש"י differ as to why the חכמים maintain that the ניקף is required to pay only for the fourth wall and not for the preceding three walls.
ורבי יוסי סבר אם עמד ניקף וגדר את הרביעית – And ר"י maintains only if the encircled party arose and fenced in the fourth wall, etc.
Summary:
The expression 'מגלגלין עליו את הכל' indicates that ר"י is more מחמיר than the ת"ק (hence all the לשונות in the גמרא [bar the last one]). The expression 'אם עמד' seems to limit the liability to the ניקף only if he himself built the fourth wall (hence the לישנא אחרינא).