You might not be able to see an invisible opponent, but you do have a line of sight to them, in the strictest sense. Under D&D rules the invisible target has 100% concealment but no cover. The party in the example of the lobbed grenades also enjoy 100% cover. If I'm an archer and my opponent gets behind cover I can't bypass it by firing an arced shot (even though this was how real life archers functioned in battles I believe). Missile weapons in D&D are assumed to travel a straight path, and grenadelikes are no exception..
Regards "Aid another"
I believe I have seen it mentioned when it comes to helping another to hide, strength checks to break doors and the like, so I don't see it as too much of a stretch. There would be a reasonable limit imposed of course. It definitely isn't unreasonable to have an assistant that actually does provide assistance in an endeveour.
Without my books here at work, it is rather difficult to remember exactly where all these strange rules are parcelled at.
I'd certainly consider using aid another for mechanisms like strength checks but I can't for the life of me think of any skills that I could conceive using it. (Which is not to say there aren't any)
The levelled members can just as easily be regarded as summoned creatures that only appear if there is a significant number of Kobolds, in which case, it is the Kobolds providing the challenge...
Trippy

Except that I don't see Sp Abil: Summon Sorc8 under the kobold's abilities. I just see that large groups of kobolds tend to include levelled individuals. It's the levels that provide the challenge. You could make those sorcerers levelled kittens (with appropriately modified somatics of course

) and they would still be the ones that pose a threat. (Plus I hear they have an attack that does 12d6 cuteness damage)