1) there's nothing technically limiting you from doing this if you really want to... but it defeats the purpose of having the system since you won't be able to access the features for some amount of time... You realize that LinuxMCE runs a MythTV instance in the background, right? You can import your stuff into that if you're tricky - I made a post about it somewhere when someone asked something similar.
2) again, this is more trouble that it is probably worth, and will not get you the results you are looking for, especially for media playback.
3) by 65xx I'm assuming you mean one of these:
http://ark.intel.com/Product.aspx?id=42805 . Yes, this has the power to do everything you're doing now with LinuxMCE, as far as I know. LinuxMCE hosts most of the services you have set up manually - Myth, samba, and mythweb are all installed for you. So is a complete network management pacage.
4) ...read the FAQ and other forum posts. LinuxMCE can use X10 devices.
You need to read the FAQ in its entirety.
http://wiki.linuxmce.org/index.php/FAQ