Welcome. I will get right to the point...
You need two ethernet cards in your LinuxMCE core, because LinuxMCE runs a DHCP server on the second ethernet card. This essentially turns your LinuxMCE machine into the router and gateway for the entire home. This was done as the best way to be able to handle plug and play of network devices.
Yes, we have a driver for the CM15A, although I have never used it. It was written by a volunteer. I forget his name now... It should find the device the moment you plug it into the core or a media director, and ask to install it. You can then add devices in the web admin, under Wizard > Lighting, as X-10 is not automatic, it can't be dealt with by the Setup Wizard.
Because LinuxMCE is controlling the devices and logic, if the computer running LinuxMCE is turned off, or suffers a crash, then the automation will not work. Bus systems like KNX (which we also support) handle this type of autonomy much better.
I would like to conclude by saying, LinuxMCE is not a media center, it is a smart home platform. As such, it has a much larger scope of features than any media center, and this also means that the supported PC hardware is smaller, because we have to manage a large set of features amongst a small group of engineers.