Why are there dots on the word "Es"?
Since it is as if "Es" was not written (See Sifsei Chachamim).
Rosh, Riva, Moshav Zekenim - i.e. the result was the sale of Yosef, which led to their sustenance during the famine. Hadar Zekenim, "Rashi" on Bereishis Rabah - they went in order to eat and drink. The Oznayim la'Torah (on Pasuk 2) explains that, like most shepherds, the brothers grazed the sheep with their own interests in mind - so that the sheep should grow fat and provide more meat and go up in price - and Yosef taught them that, until a sheep is slaughtered, one should graze the sheep with the good of the sheep in mind (like the Midrash with Moshe Rabeinu and sheep that ran to the water to drink - See Oznayim la'Torah 2,. DH Eleh). And the Torah teaches us this here, to explain why Hashem chose Yosef over above the brothers, because Hashem tests the leaders of Yisrael to see how they treat the sheep - like He tested Moshe and David - and we see from here that Yosef, out of all the brothers, proved himself worthy of leadership.
What is the significance of the fact that Yosef's brothers were shepherding the sheep specifically in Shechem?
Seforno: Because it was not far away. 1
Rashbam and Targum Yonasan (on Pasuk 13): He wanted to check on their wellbeing. Since not so long ago they wiped out the entire town, it was a dangerous place to be in.
Oznayim la'Torah (citing the Midrash Rabah): They went to graze the sheep in Shechem, because Yakov had purchased it and they refused to graze in a location that did not belong to them.
Malbim: It was far from Yakov's house. They trusted that Yakov would not send Yosef there, for it is dangerous. Also, if they would kill him, perhaps their father would attribute it to the city dwellers, since they hated Yakov (since his sons wiped out Shechem).
Why must it say "[Tzon] Avihem"?
Ha'Emek Davar: This teaches that an Aveirah leads to a bigger Aveirah. They grazed themselves (ate) from their father's flock without permission (refer to 37:12:1:1), and came to a much greater Aveirah. 1
Harchev Davar: Chulin 4b says that enticement is only through food and drink.. They never would have done so had they not gone to graze themselves!