[Acolyte]

...the issue of what IN is and isn't meant to do should be decided by the gaming group and facilitated by the rules, not the other way around.

It's necessary for every non-universalist game to make assumptions so that the game can actually be played. IN doesn't well handle a game where humans know about both angels and demons and fiery sword-wielding angels do battle with fighter jets. To play that game, you'd need to make changes to IN mechanically. Less hyperbolically, to play truly Backwards IN, you'd need to make changes to game mechanics to make that work.

When and if a 2e ever gets written, my hope is that the core rules will be less human-antagonistic, whether or not human PCs are mentioned in the core book...

I don't think that's a bad thing at all, especially taking human PCs out of the core and making the CPG actually about playing humans instead of creating some more NPC types. In fact, I wouldn't mind seeing the approach be much more like that of Ethereals in IN currently--nowhere near enough mechanics to play them without the special book, and enough disconnect between the special book and the core to make that option very distinct from the baseline. Any player wanting to play an Ethereal currently knows his character will not fit in, mechanics or fluffwise, and any GM agreeing to allow an Ethereal PC knows that he has to do some work to make sure that the player gets as much out of the experience as the players of celestial PCs.

In review, that's pretty much how I really see human PCs in IN currently (both fluff and mechanicswise), although I am aware that the core doesn't make that distinction as sharp as it is in my head. (The distinction is there, but the fluff and mechanics would have corresponded more tightly had the book said 'don't play humans or expect to play them, but here are some ideas that might help GMs who want to try it', rather than saying 'humans are subpar but you can play them if you really want to'.)

...[the core book] assumed that all games would be what is now referred to as dark, low-contrast games.

I'm not sure if I agree with that last bit. Night Music is bright/low-contrast, and most IN games I've played or been involved with have been bright/high-contrast, without that being much of a problem. (The next most common type is, I agree, dark/low-contrast, because most demonic resonances and attunements are so blatantly exploitative that it's not particularly easy to cast them as 'good guys' or even 'plucky antiheroes'--it's much easier to drag angels down to their level.)

How many styles of play should IN support? At some point, the game designer has to draw a line (shooting missiles at angels is on the other side of that line, probably). That "gamebooks should respect and support" any type of game players can imagine is a pretty impossible requirement for any game to meet. Rather than have IN (or any RPG) change accommodate all imaginable playstyles, I'd rather it fix the problems within the narrower set of playstyles the game supports well.

Multiple play styles should be facilitated; and a version where humans take center stage and celestials play a supporting role should be among them.

That is your opinion, much as it is my opinion that such a playstyle doesn't have to be supported to make IN a fun RPG to play. Diversity of playstyle isn't always a good thing, especially when adding support to a divergent playstyle weakens the themes and efficacy of a primary. And I am not saying or implying that would be the result of all attempts at making humans (or even human PCs) more powerful in IN. I would have to see the full rules on such to make that judgment.

This is not a debate between mature role-players on one side and hack-and-slashers on the other; so please don't trot out the usual anti-H&S arguments.

I don't think that's what Azel meant; I took that to mean that IN has an unusual assumption among RPGs (though not unique) that that PCs don't spend the majority of their time together. Since the default IN assumption about celestial density puts only a few celestials of the same side in every non-metropolis city, and the game setting has even allied celestials focusing on tangential goals with their earthly time, the PCs tend to only operate as a group at times of crisis. Mostly the Trade angel works on revitalizing the downtown, while the Stone angel assists the downtown gang with protecting their neighborhood, all while the Fire angel immerses himself in hotspots of cruelty. They try to outmaneuver their infernal counterparts, and they pool their talents and contacts when a new demon arrives or a wild disturbance interrupts their monthly breakfast meeting. (The latter is of course IN's version of fantasy RPGs' 'you're all sitting in a tavern...'.)

Now, you don't have to play IN like this, but that is certainly the default expressed in the opening vignettes and with NPCs such as those in Night Music--and by default, I mean default campaign. A one-shot (or multiple session minicampaign with a planned arc) is much more likely to be "your Superior assigns you to this group and you are tasked with this mission", where you would have all the PCs spending all their time together (like the PCs' role in Night Music--this is of course more analogous to an adventuring party, but it doesn't have to convey implications of hack'en'slashery, because the mission could just as easily be one of espionage or politics). And again, it's certainly not the only campaign model that the rules or even the fiction support (a Triad of Judgment is a solid campaign setup and they spend all their time together), but it is the default.

One advantage of that default is that the "spotlight time" "play balance" you mention (which is, I agree, very important even in any RPG) is easier for the GM to handle. While I have no experience GMing human PCs in IN, more than a few PCs have had human Servants, and creating plots in the more episodic model to showcase those humans was not too difficult (and was important--creating encounters and plots where the servant was useful pleased the player who had spent all-to-rare CP on the servant, giving that player more spotlight time even if it wasn't his PC who was being most useful). And I am talking about times when the humans' rolls came into play, too, not just this-is-how-it-works-on-Earth scenarios. For example, one game had three angel PCs, one of whom had a full Soldier (statted normally for a starting Soldier) as a Servant. Because the angel who had the servant was a Seraph who (RPwise) had a very hard time dealing with humans, the servant got about one-half of the Seraph player's spotlight time, something the player was very happy with (the two characters had a sort of nobleman/valet relationship). It was not difficult to construct an episodic, assumed-downtime, default IN campaign that showcased the servant mechanically one-sixth of the time.

...the main issue I have is the severity of their inferiority, to the extent that you have to hack the rules or largely dispense with them in order for humans to be viable. And I'm especially bothered by people not even bothering to look at the issue, because they're "only humans" and thus not worth the effort.

I don't see bringing human PCs up to celestial PCs in power as an important or interesting goal for my enjoyment of IN, the same as I feel about Undead PCs, Ethereal PCs, Remnant PCs, Reliever PCs, and Demonling PCs. I don't think that the way I play IN is the only way to play the game, but I do see the potential for diminishment of the game down that road--simply adding more Forces dramatically changes how humans respond to resonances (it would get even more difficult to play a Kyriotate, for example, and nearly all demons would have a harder time on Earth), how they behave in their own dreamscapes (the Marches, even within the Vale of Dreams, just got a lot dicier), the feasibility of the War remaining a secret (no more Ofanim spiraling across the world at will, because the average human's Perception just jumped), changes the default for what an Agility of 6 or a Precision of 4 mean (with average humans having higher numbers for each stat), makes Songs more accessible to humanity (which could be a good thing if that's what you want, but that cuts into celestial uniqueness), and so on.

If you say that only PC humans get more Forces (thus sidestepping some of the sweeping implications), you 1) aren't addressing the underlying concern that RAW humans simply don't work, so those problems remain for NPCs, and 2) are butting in on the "different than the average person" appeal of playing a celestial in the first place. No, these high-Force human PCs can't get killed and buy a new Vessel to return to Earth, and they don't have resonances, but they are superpowered people, and IN already has supers--they're called celestials.

It's not that I'm not bothering to pay attention to the fact of human weakness, it's that I haven't yet seen a significant reason to overhaul IN to address it, when the quick fixes work well enough for NPCs and the focus for PCs is placed firmly on celestials.