I think that MG's are easy to beat, once you know how to do it. Plus once you know an MG's location its easy to call out to your team so you all don't run in and get wiped out, or if you do you can wait for your team mates to respawn and try another approach.
It depends. If the defending team could defend outside of the sewers with the MGs, they could stop every single rush, picking people off as they come up the ladder. There could be some kind of restrictions as to how long people can stay in place without moving up. It is a rushing event after all.