first thing to clarify is: triplehead IS NOT multiscreen in that sense. the triplehead represents one very wide screen. that is how the system and modul8 see it. the splitting happens inside the triplehead box and has nothing to do with modul8's advanced output capabilities. which are designed to span the output over several graphics cards, and only in this scenario the quicktime problems are in effect.
by the way, 7.1.3 is the version where even these problems aren't present. it started with version 7.2 if i recall correctly.
so no worries about that, at least not in that context.
well but your problem sounds weird. i used my triplehead2go analog edition on a similar g5 machine and my macbook pro and never had such a problem. i'm intrigued by your mentioning of a "matrox control icon" in system preferences. did you install that manually? i never used any driver or other additional software. maybe that's causing your problem. (hopefully there is a way to remove it completely) just plug it in and set the resolution/refreshrate through the standard display preferences. a common problem i encountered were certain beamer/cable combinations that didn't handle the 3072x768x75hz setting, switching down to 2400x600x60hz always solved it. also don't use any other cable then the short one supplied to connect your triplehead to your graphics card.
regarding software versions i'm used to be always up-to-date and see what happens. i'm running with the triplehead since osx 10.4.8 and qt 7.1.3 and modul8 2.5.1-2-3 i think. on the way with almost any combination uptil now where i'm using 10.5.2, 7.4.1 and 2.5.5. always worked fine, both on g5 and intel.
hope anything of that blah blah helps you