If a magus were to use an Intellego Imaginem spell to scry on a mundane, and that mundane were to interact with a demon... would that demon be able to deliver false information to the spell? If so, to what extent? Would he have to be aware of the spell? Wondering how this situation would work...
Well, for starters, the other party may not even appear to be a demon and could look and sound very much like a mundane.
I think you have multiple possibilities here for what is detected (and I use that term cautiously). The demon might be able to mask the entire conversation for both parties so it appears completely innocuous. The demon might be only able to mask his portion of the conversation, so you need to consider whether the demon has enough power to lie to make his fake parts of the conversation blend in well and be congruent to the other speaker's portion of the conversation..
Demons are not all knowing, or even especially well informed. You might catch the demon off guard - they wouldn't necessarily have any information that you are scrying on them, so not be in a mundane shape to fool you.
If you use an InCo spell to scry on what you think is a mortal, even if you have an arcane connection, the spell may not work, especially if the demon is not in corporeal form at that moment; at the same time, an InCo spell may just reveal the apparent mortal. InIm spells should just bring images to the person or thing or place, so you might get anything - images of whatever the target is doing at the moment. InVi scrying spells should bring images of the demon so long as it is entirely unaware of the scrying, but it isn't trustworthy at all.
If the demon has a reason to think you're scrying, or if it is in a disguise at all, or if it is in a form normally undetectable, you won't be able to detect the demonic aspect. As I recall, the demon's Might score should alert it to the scrying if you fail to penetrate, but I'm not certain off the top of my head.
In my current saga, this situation has come up.
A person, under the influence of an infernally sourced effect that could be called an infernal illusion (equivalent to a CrImg to change your appearance to someone else's), hired an underling to break into one of my sodales' home in the town of Foix. As a result, via a series of roundabout measures, we have an arcane connection to this person.
Using this, my sodale used an Intelego Corpus spell linked to a Creo Imaginem to see what the person looked like.
The infernal effect, our SG decided, caused a flicker as it overrode the intelego effect, resulting in us seeing the true form for a couple of moments before it was overwritten by the illusionary form (as an infernal effect will read as true to hermetic magic).
Fortunately, I have a remarkably good memory in character, and so was able to use a CrImg of my own to create a short duration visual image of the face we saw before the effect overwrote it. I really need to find a text on art of memory to improve that skill, though.