To address specifically the idea that "Other creatures easily succeed on the save, but the allies know the effect is beneficial so choose to fail the save":
First, other creatures would not necessarily GET a save to begin with, assuming this would be a figment, since that requires interaction beyond just looking at the illusion.
Second, a creature that disbelieves an illusion can still see it. I am not certain as to how they would perceive indirect consequences of an illusion (i.e. illumination), but I would interpret this as meaning "even if everything else worked, it still wouldn't be very stealthy".
Voluntarily failing the save is less clear, so let's just say that part works for the benefit of doubt.
The next question is whether such a spell would be an illusion in the first place. I am not entirely familiar with Pathfinder, but unless there is an existing illusion spell which clearly states that it can be used for illumination, I would conclude from "a spell which creates illumination exists in the evocation school" and "no spell which creates illumination exists in the illusion school" that a spell which creates illumination is more appropriate as an evocation even with the other desired features. This might or might not be relevant though.
Based on the way Illusions interact with concealment, I would probably say that having a normal Light spell, then covering that Light with an Illusion that blocks it, would allow for those that disbelieve the illusion to see the illumination as well; it's not entirely logical to me, but there's a reason it's magic. Perhaps a combination of those two effects into a single spell would be valid; if the Illusion spell required for that effect is higher level than the Evocation spell (Light) required for that effect, then I could be convinced that the resulting combined spell would be "more Illusion" and fit the Illusion school despite also requiring Evocation. This would be my solution if having the spell be an Illusion is critical. Otherwise I would probably call it Evocation and base details like spell level and duration on the Communal Darkvision spell mentioned by Hey I Can Chan.
In conclusion, spells which allow certain characters to see while other characters receive no benefit already exist, and depending on why those aren't sufficient there could be other solutions. But without knowing why the existing option(s) aren't desired the particular inadequacy can't be intentionally addressed. As for the specific mechanics proposed in the question, I would say no an illusion spell which provides illumination to people who fail the save, even if they know it was an illusion in the first place, and does not provide illumination to others because they succeed an intentionally easy save, does not make sense within existing mechanics.
TL;DR: The spell as described does not sound legit, but the desired effect is probably possible some other way.