well the difference is this T Wrecks:
the railing are an Alpha texture. the railing is in it but the rest is 255 blue.......and when turn it into a func_illusionary, you set render amount to 255 and then you cant see the blue part ingame.
func_illusionaries are ALWAYS passable, hence "illusionary"
the curtains are a normal texture, func_illusionary with no special attributes.
what makes the railings not passable is a brush made to the same size as the railing which is invisible to the player, but blocks his movement through it...its a "clip brush"
a railing could also be a func_wall with render amount of 255...but then your bullet decals will be placed in the "air" between the railings. with illusionaries the bullets go through......see......you cant shoot through a func_wall, unless its thin.....but you still see the bullet decals(even blood in this case
i hope you get my point.
i dont think you can do anything with bots on this curtain because we just solved an issue where bots cant SEE through some things......and this curtain thing is falling into that fix.....i think
i also cannot answer the light/dark issue either