1) R' Ami & R' Asi say if one has to ask another person for help he is judged with fire and water. What exactly does this mean, that the embarrassment is akin to Gehinnom?
2) Regarding not being mispallel achorei beis hakneses, I'm unclear on how the logistics work, especially according to Rashi. Were the doorways on the east side and everyone faced the door during tefilla? If so, why would someone come in and deliberataly face away from the door toward the congregation? Also, how is this "achorei" -- the door is actually at the front of the shul!
1) Yes, when one is so destitute that he must go to such an extreme measure as to rely on the goodwill of others for his basic needs, it is a very embarrassing experience. In fact, our Sages explain that this is the reason why Hash-m created the world in such a way that we must do Mitzvos in order to earn our share in Olam ha'Ba. He could have created the world in such a way that we were immediately granted eternal life, but that would have been "Nehema d'Kisufa" ("the bread of shame"), since it was not earned by us. Therefore, Hash-m created the world in such a way that we earn (or at least it appears to us that we earn) our share in Olam ha'Ba.
The Vilna Ga'on adds that when a person becomes very embarrassed, "the red [color of his normal complexion] goes away and white [color] comes." In this way, embarrassment is compared to being exposed to fire -- which is red, and to water -- which is clear/white.
2) According to Rashi, the entrances to the synagogues were on the east side, which was at the back of the synagogues, since the synagogues faced west (from Babylon to Israel). The people praying stood facing west with their backs to the east (to the entrance to the synagogue). One who prayed "behind" the synagogue was thus praying on the eastern side, and was facing away from the synagogue (towards the east), with his back to the synagogue (to the west).
Tosfos (6a, DH Achorei) had a different reading of Rashi, and thus had a question on Rashi's explanation. Tosfos' version of Rashi said that the back of the synagogues were on the western side, while the entrances entrances to the synagogues were on the east (thus, even though the people came into the synagogues on the eastern side, and they faced away from the door towards the west, the east was still considered the "front" of the synagogue). Tosfos argues, because according to this explanation, if one stood behind the synagogue (that is, on the western side) and faced away from the synagogue, he would indeed be praying in the same direction as everyone in the synagogue, and it would not appear that he was rejecting the One to Whom everyone was praying! Tosfos therefore explains that he was standing on the eastern side, which is basically the same explanation that appears in our version of Rashi.