I have a premise (in detail below) in which I would like to set up some doubt in my main character about his own sanity. I think the situation is such that it's easy to make my main character doubt himself, but I find it's much harder to make the audience do so.
Generally speaking you know no matter how insane the situation is your main character will always be sane. If he is hearing voices and seeing things they really exist, if others tell him that his best friend never existed he did etc; the main character is always sane... In all the stories which tried to tell my my main character was crazy I am usually bored because I don't believe it, and without that uncertainty the plot seems pretty bland and overdone.
The only time I saw a story do a remotely interesting take on a possibly-crazy character was Final Fantasy 7, and I think it helped that he was actually crazy, just not in the way others were trying to make him believe. I'm looking for solutions like this, ways to modify the world and story to make the possibility that something isn't right seem a possible outcome to the audience, not questions of writing style or character depiction (ie, I want to modify the world, not how I describe the world, to make this more believable).
So I'm looking for advice on how one would to make a character seem less-then-sane in such a way that the audience may consider it. I'm happy both in general advice, and specifics to my case.
Our main character's home farm is invaded by bandits planning to kill his family and pillage when a stranger appears in random light, mutters about wrong coordinates, then saves our hero, but not his family, before disappearing again in an explosion that knocks the hero unconscious.
Some time after the hero's leaving his destroyed home our hero wakes up in 'the future' and learns the stranger that saved him was someone trying to change the past, but who had to do so with unknown technology and ended up showing up at the wrong place in his only possible attempt at time travel. As a side effect of the accidents that occurred during that time travel the hero is now able to travel between the two time lines (without control). He died in the original timeline, and is now the one outlier between the way history had happened the first time and his current timeline; making him the only one able to avoid the outcome.
This is made harder when the eventually learn that the two timelines have split, changes to the hero present don't change the future timeline, and as the hero changes his present it drifts from the future timeline enough that their knowledge of the past is of limited use.
At a later point someone is going to suggest that the hero has never time traveled, the story is nothing more then a fiction made up by someone who couldn't handle the trauma of his home and family being destroyed. They claim he survived because he ran rather then try to help his family, and he created the fictional time traveler to explain his survival without his being a coward. He made himself out to be the most important person in his time (as the one outlier able to change the bleak future) to handle his feelings of uselessness at being unable to prevent the attack. It's suspicious how every time something bleak happens in his present he goes to the 'future' and plays hero, his mind compensating for bad situations by giving him a story where he was a hero elsewhere etc etc. The point out the fact that he can't predict the future, since timelines have diverged.
The problem is that the audience will never believe it, they will always accept that time travel is what is really happening. If I can't make the audience contemplate the possibility that something is wrong with the hero any drama I pull out of the "am I really sane" question starts to feel more like angst.
I toyed with making him actually be a little ...well not all there, that the process that pulled him between the timelines caused some bizarre effects on him, he sees things that aren't there because he's seeing glimpses of other potential timelines etc, but no idea I came up with felt interesting and not too distracting from the other plot points I planned out. I can't think of something that twists his viewpoint sufficiently to leave the audience willing to believe something was wrong with his view of the world while still leaving him 'sane' enough to continue a story that has many other themes and plot points that I don't wish to distract from by having the audience questioning every thing the protagonist perceives even in the present.
I'm looking for ways to make it plausible that the alternate explanation, that he is imagining things, is possible. I suspect this would involve actually messing with his perspective of reality, such that there is something legitimately wrong with the vision the audience sees, thus suggesting a scewed perspective from the protagonist. My question is what can be done to mess with his perspective or the world enough to make the audience agree that something is wrong without making him certifiably insane? Or are there other ways I can mess with the timeline to make him appear less believable?
I imagine by 1/2 to 2/3 of the way through the story I would like to provide evidence which strongly suggests he is sane, or at least that the audience can trust what happens on screen from here on out is really happening, so I need something to cause confusion early on but can be explained later.
I'm pretty open for any way to justify some real uncertainty in his view of reality as a time-traveler to the audience.