"Almost"?? What doesn't work?
10.4 is the only version that will allow OS9 Classic. That doesn't really matter unless you happen to have OS9 or older software that you want to "just drop into" sometimes… i.e. older databases, word processors etc. It is also the fastest-running OSX on that machine - note when I say that, I mean a teeny-weeny bit faster. There are some who obsess over that and will shit on 10.5 all day… I say phoo.
Note that both 10.4 and 10.5 will run the TenFourFox browser and that still kinda works.
10.5.8 is the most complete OSX that will run on the MDD.
Again, Nottalotta difference - either one runs really well on your machine.
The 10.6 you mention is really 10.5 with a few chunks of 10.6 code replacing the original PPC. Note that 10.6 (Snow Leopard) was another stopgap system to help ease the transition from PPC to Intel procs on early Mac Intel-equipped hardware. It's of interest only to wannabe developers and hackers. I have never seen a single comment from anyone who describes it as actually being superior to 10.5 on a PPC in any way. Most are just happy to actually see it run and play with it.
And yes, you can still boot 9.2.2 - It's best kept on a separate volume or entirely separate drive the way you have it. Despite the temporary existence of "Classic" the reality is that the "old" Mac OS and OSX don't get along …… at all. OSX is UNIX-based and OS9 and all older Mac OS versions simply can't comprehend it.