I will have a go at answering some of your questions if I can. Others will probably add somre more...
1. I run a stupidly confusing network. It would be far too difficult for me to modify my wired connections and basically it just involves a lot of questionably placed switches (not routers or hubs). To my point... I am wondering if LinuxMCE has an easy to configure/use/modify firewall in it? Right now I have a really nice small Smoothwall box as my router/firewall/DHCP/TFTP system and am wondering if LinuxMCE will be a drop in replacement for that (mostly wondering if it has an easy to work with firewall) or if I'll be shutting off *some* services but not all and running the 2 devices side-by-side?
Your CORE machine will act as a gateway and really prefers to operate with two network cards. By default it will want to act as your firewall/DHCP server and TFTP server. There is an easy to use Firewall admin screen (part of the standard admin server) that has worked well for me. So basically it should fully replace the smoothwall server (which is exactly what I did).
The only thing to be careful of is that most of the standard network configs (ie: your /etc files) are extracted from the database at each reboot so changing them directly is typically not effective. They can be directly changed in the database or alternatively (and preferably) altered through the Admin screens provided.
2. I have a pretty decent MythTV arrangement. A single master backend and several frontends one of which is netbooting. Will I be able to move the single master backend to a slave backend to keep all the recording data intact and not have my recordings turn into their filenames? Will the netbooted frontend just *work* once I install MCE (assuming I just disable my Smoothwall setup all together?)? Basically, will I lose any of my existing MythTV functionality? I'm hoping for all gravy and no lumps.
This one might be better for MythTV forums, but I would be looking at seperating them completey to start with and migrating your current backend data to the new master backend (CORE machine) and getting everything working and then look at adding the old stuff in. I am still not happy with my MythTV setup but have not put much time into it as the Wife is very happy with the current TIVO setup that is operating and until I can offer something better the cable box etc will stay attached to that.
With your experience with MythTV it sound like you should be able to add some real value here.
Standard frontends should still work fine with the CORE as a backend.
Your netbooting machine when added to the Pluto network should just boot up as another Media Director.
3. I have just a little bit of confusion on connectivity. I have enough Bluetooth adapters for all of my systems. Does this mean I can forego any IR or Serial arrangements? I see that a specialized (somewhat expensive) switch that includes IR and Serial ports is suggested (something like GS-110 comes to mind but I can't quickly find the wiki page that talks about it). If I go all Bluetooth (just for control, I have wired ethernet for connectivity) does that mean I can ignore any IR info? I do have a projector as one of my displays and it has a serial connector on it (which I assume means I can have my LinuxMCE system in that room control it) but for now I am not interested in that (awesome) feature. Is that about the extent of why I would want the rather expensive serial/IR/switch device? If I don't include it now, will it be painfully difficult to integrate later?
The bluetooth adapters are at the moment only used for Mobile Orbiter communications and "Follow Me" type functionailty (working out what room you are in). In theory they could also be interface devices for other devices (much the same way IR is used) but to my knowledge it is not doing this at the moment.
The question on how to control your passive (non IP) devices is really down to how they can be controlled. IR seems to be the lowest common denominator. A GC-100 may be what you are talking about, but they are not that expensive. If anything requires IR control then you will need an IP controlled IR transmitter of some sort. There is a few options, the GC-100 is one.
4. Light control. Looks very awesome! High "wow" factor, but low usability factor, for me personally. So I'm wondering, (subectively of course) if it's worth it. How difficult is it to get it working, what are some of the costs I might expect, and do any lights work (is it a plug replacement or a bulb replacement or what...)? I briefly have read about Zwave but nothing difinitive really seems to be available to the average joe consumer with a lot of time, a little bit of extra money, and a desire to learn.
I am still in the same situation here. I am waiting until ZWave becomes a bit more available in Australia - but from what I have read you should be able to replace your light switches, add a USB controller to your network and go from there. Much the same as setting up X-10 I imagine but signals are sent via RF rather than over the wires.
Anyway, hope I have helped a bit.