No worries - I know everyone here take their audio VERY seriously.
Center channel is a much discussed topic among those who mix in 5.1. As a complete novice at mixing in surround, I spent a lot of time auditioning various mix approaches, isolating channels, etc. to see what was really going on. I found such a broad range of approaches that I decided there really was no hard or fast "rule," especially regarding vocal placement. Avalon had some fairly radical isolation (all vocal processing in rears, multiple lead tracks in fronts, totally dry vocal in center), whereas many other mixes tended to avoid the center channel altogether. I also noticed that placing audio in the center channel sometimes resulted in some odd phase issues that seemed to really vary between different listening environments. I'm sure the members of this forum have carefully calibrated their playback levels and positioning, but this is certainly not the case for a broader set of listeners who have widely variant set-ups. The safer bet was to avoid putting too much crucial information in the center channel. I did take advantage of the center channel for a few subtle moments. For example, the vocal on the verses of "Devil's Stomping Ground" begin aimed more in the center, then spread to L/R each time the melody ascends.
I agree that with a little hindsight, the mixes could be a little bolder in localizing some placements for more dramatic effect. I'm very happy to hear that you are finding the music/audio to be of high quality. Despite very low budget, and mixing conditions that you would simply not believe (on my laptop, in a very poorly converted 10' x 15' office space with no isolation whatsoever), I think it came out well. Everyone involved certainly strove to achieve as high a quality of performance and recorded audio as possible. Of course working with pros on the mastering/authoring end made a world of difference (though I was really psyched that mastering engineer Adam Ayan found he had to do very little sonic adjustment to the mixes).
I'm absolutely open to constructive criticism and value the feedback from the type of dedicated listeners that actively participate in the forum.