1. As stefanero said, this problem isn't a problem with servers banning aMule. There's a bug, apparently in Apple's code, which causes aMule to stop accepting the connections that other computers are trying to make at port 4662.
3. Running two mules on one network connection will work fine, but it is somewhat inefficient. First, the two aMules will compete for your bandwidth. To avoid total saturation of your line, you will want to limit each to only a portion of the line. However, there may be times when one isn't using all that it's allowed to, but the other won't be able to "know" that and take advantage of the slack.
Also, you are making inefficient use of the credits you're earning. One of your clients might be trying to download from a source where it has no credits, while your other client has been uploading to that source and has earned many credits.
If you can run all of your downloads from one computer, you will solve these problems and maximize the efficiency of your aMule.