Religions that can only be found in Ditillio's notes don't count. He was off the show by the third season and that was the end of any of his contributions. His extensive writing about the races in the Non-Aligned Worlds is interesting, but ultimately irrelevent to the back ground aliens whose highest point of on-screen development was the very on-the-nose bit about Drazi infighting. You mention alternative Narn belief systems, but the differences between these were rarely mentioned nor explored on the series.
Again not entierly accurate; material from Ditillio's notes does crop up later on down the line, though mostly in terms of background and subtext. I suppose the more prominent examples are in Season 5 with 'Day of the Dead' that briefly touches on the Brakiri's "mystic" religions and the Hyach/Hyach-do episode (name escapes me) with the age dominated government and even the name of their deity comes right out of Ditillio's work. So while it wasn't a subject often visited, that material wasn't forgotten and served it's purpose behind the scenes as a background guide. The reason it was never really focused on is quite simply because the show wasn't about all of these individual cultures. From the end of season two, all the way through to the end of season four the arc really had to take centre stage, leaving little or no room for little stand-alone stories. But just because those cultures and their internal diversity weren't thoroughly explored doesn't mean it wasn't thought about.
Take the Drazi for example, similar to the Minbari they had several different modes of dress from the grey robed followers of Droshalla, to the more elaborate and colourful, uniform like costumes with the big red shoulder pads and the quilted tunics worn by some of the diplomats and merchants.
I can see more diversity in architecture walking down my block, and there aren't any houses here older than fifty years. Your point about Minbari dress is well made. A quick Google search quickly confirms there was more variation in dress than I had remembered. Although, apparently not that much variation in dress since the time of Valen, apparently. The point about there being no variation in Narn dress still stands.
As far as Minbari architecture is concerned, the reason you'll find so much more variety where you are is because (among other things) you're not on Minbar.
This is a culture that prides itself on preserving their buildings with a deep respect for ancient traditions and styles. Hell, they even went to the trouble of maintaining Valen's residence EVERY SINGLE DAY for a thousand years, so when Sinclair moved in, it felt to him (ironically) as if Valen was still livign there.
In that kind of culture and especially in culturally important cities (Yedor being the capital & Tuzenor being Valen's "favourite city") you're not going to such much in the way of new development. Which in fact says allot about Minbari culture as a whole since they had effectively plateaued and sort of fallen in to a stagnant decline. In essence echoing the Vorlons with their fixation on order and sameness; "frozen in time" as the Shadows pointed out.
As for the Narn sure the wardrobe didn't vary much, but to be fair, this is a culture that had in the recent passed suffered genocide, oppression and a systematic striping away of their cultural identity that effectively forced every man woman and pouchling to unite and become fighters. So the Narn we see are a product of that and again, like the Centauri, most of the Narn we DID see were either leaders or military types. I think we only ever saw a handful of civilians, one Narn child and a grand total of five Narn females (two of which were Na'toth.

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That was a cheat to re-use the set, as admitted by JMS. Thus the line about budgetary constraints in my original post, although, in the show's defense, JMS came up with a rather clever cheat in this case.
As you say, it was because of a lack of money, not through a lack of will or imagination and as I pointed out, it was directly addressed in the script so it hardly counts as proof of a lack of cultural diversity.
It's a fair point that we never saw a wide cross-section of Minbari (which makes it impossible to critique the Centauri, when I stop to think about it, because we saw so little of their society outside of Emperors and Ambassadors), although we got a good look at their civilization in the episodes dealing with the Civil War in season four. And in those episodes I don't see the diversity of a real, living society.
All we saw of the Centauri was the ruling culture, the Royal Palace and the Capital's skyline. That's like visiting Washington DC and saying that all Americans wear suits & ties while living in white stone houses with bloody great white stone monuments and large rectangular ponds everywhere.
The scope of the show simply didn't have a lot of room for the full breadth of Centauri culture. Indeed, given how many distinct cultures were seen on B5, it's amazing we got even as little internal diversity as we did.
For one, Delenn's assesment of humanity in that short speech is too simplistic. Humanity pushed to build the Babylon station. But they strove to broker a peace because they had been decimated militarily, and can probably be blamed for the terrorist attacks that destroyed the first three Babylon stations. Just as many humans fell into line behind a xenophobic dictator (Clarke, who was against building communities) as opposed him. There's no universal human trait to be found there.
Over simplified perhaps, but what do you want? A four hour dissertation? These episodes only last 40 odd minutes so a character making a general observation about what strikes them most about human culture really has to get to the point. Anything more and you're going to send the audience to sleep.
As for all the aspects of human behaviour you listed, I don't recall Delenn saying that humans form conflict free and harmonious communities.
When "Believers" comes up in conversation, I frequently mention how it is the episode that very nearly turned me off of the show altogether.
Being a practicing Christian (I'll get it right one day), I'm always interested in sci-fi series and the way they treat religion as a whole, Christianity in particular. This episode at first seemed like just another anti-religious tale, wherein those who practice a religion are portrayed as nothing more than superstitious nutcases. But I stuck with the show, after a couple of friends urged me to do so. Bear in mind I had not yet seen "Parliament of Dreams". Once I had, I was convinced that there was no animosity against religion or religious people themselves. It gave a fair treatment by showing a variety of angles and roles played in the lives of various characters and cultures.
That's probably why I'm not as angry about Believers than I was before. Overall, B5 handled religion quite well and fairly. This one, speaking as someone who does go to church each week, just felt over the top I guess.
I find it interesting that of the many criticisms I've seen levelled at this episode over the years (it's not my favourite either, just to be clear) the perception that it has an "anti-religion" or "anti-fanatic" messages only seams to come from practicing religious types.
Maybe it's because I'm a devout agnostic (practices Tuesdays & Thursdays when the weather's nice) but my perception was that the show was partly about a lack or true respect for another's beliefs. Franklin decided that being a doctor gave him the moral authority to disregard one of their most fundamental beliefs because "he was right and they were wrong". Sure, from the outside it seams silly and superstitious to believe that minor surgery can take away a person's soul (even if you believe in it's existence in the first place) but to the Onteen it was very real.
In this instance there was no "right" course of action. The kid was doomed and unless Franklin was prepared to resort to kidnapping there's no way he could have lived.
Like it or not, nobody has the right to tell another culture what they can and can't believe. It's been tried many times before and it rarely ends well.
If Kemmer died, it would have been nice to hear some sort of reaction from Garibaldi. As it stands, Survivors is an entirely throw-away episode for me. Garibaldi's alcoholism is already established, and better tackled in later episodes. And Garibaldi's relation to Kemmer is never mentioned outside of the confines of the episode. So what's the point?
Though I agree it's not a very strong episode, it does at least shed some light on
why he's struggled with alcoholism and it's connection with his chequered history of being bounced from one job to another.
As for Kemmer; I doubt she was on Earthforce One at Io. We saw her travelling ahead to arrange and oversee security arrangements, it's not as if she was the President's personal bodyguard.
Still, personally I'd peg her as the Nightwatch type so she could have spent a chunk of the Clark years oppressing Marsies and arresting news reporters...Or I suppose she could have gone the complete opposite direction and ended up arrested for disloyal thought crimes or defected to the resistance. Either way the Earth Alliance is a big place, plenty of cracks to fall through.