1) The Oznayim laTorah suggests that the rock that Moshe was given to sit on during the battle with Amalek, "vayikchu even va'yasimu tachtav vayeishev aleha...," is symbolic of the rock / even we read about earlier in the same parsha in the shirah: "tipol aleihem aimasa va'pachad bigdol zero'acha yidmu k'even..." Shiras ha'yam was a prophecy of the fear that would overtake Amalek and our future enemies in battle.
My wife suggested that the rock / even that Moshe sat on may allude to another rock. When Yaakov is fleeing from Eisav, en route to Lavan's house he stops to sleep and rests his head on an even. Chazal tell us that there were actually many stones there, but they united into one stone under his head to protect him. Yaakov then takes that rock, "vayikach es ha'even asher sam m'ra'ashosav..." and he makes it into an altar to give thanks to Hashem for the promise of protection that was given to him in a dream. It is this promise of protection from Eisav, from future enemies, which Moshe drew upon in our encounter with Eisav's descendants, Amalak.
2) I haven't sorted out this sugya myself, but wanted to pass along the mareh makom. In O.C. 396 the SA quotes a din based on "shvu ish tachtav" (tachtav = shiur of personal space) that a person who walked outside the techum is free to move around 4 amos, as that is the definition of their personal resting space.
These 4 amos are measured relative to the person, i.e. if the person is a basketball player, then he gets more space to move around.
Flip back to siman 349 and there is another din based on the same pasuk of "shvu ish tachtav" that you can move an object around within your own 4 amos of personal space even in a reshus ha'rabim (interestingly there is a daas yachid of the Raavad who holds this is only b'sha'as hadechak, not lichatchila, either because moving a lesser amount is a chatzi shiur or because you may come to go past the 4 amah limit. The other Rishonim may hold that 4 amos is not a shiur, but rather is part of the definition of the issur -- anything less than "tachtav" is qualitatively not part of meleches Shabbos).
Here too, you would expect the din to be that the 4 amos are relative -- if you are a basketball player, you get more room. However, the M.B in Shaar haTziyun #3 quotes the Pri Megadim who writes that this is true only if you move the object bit by bit, but if you move an object all 4 amos in one shot, then the 4 amos are "beinoniyos," they are of fixed standard, average size.
Why should this din in siman 349 be different than the din in siman 396?
(MB asks the question and rejects the Pri Megadim, but it still needs a hesber.)