1. Accessing the SCSI port from the Atari side is theoretically possible. MagicMacX comes with full documentation and an example how to write a plugin (using XCode). In order to access SCSI you would have to write your own plugin and a harddisk driver (like hddriver) that runs on the Atari side and communicates with the Mac side via your plugin.
2. Internet would be the same, i.e. you would have to provide a plugin to implement any API you need on the Atari side.