The bit about psychosis is why I wouldn't support this applicant acquiring guns. As for the bit about depression... aren't we the ones who always say "if there's no gun available, they'll just get a rope."? I have also heard it said on here that the statistics back this - overall suicide rate remains stable, though firearms suicides decline.
I have a clinically depressed friend who would like to own firearms. He's been a died in the wool liberal since... forever. I took him out skeet shooting. Now he's beginning to see the light.
I certainly won't lie for him, and have encouraged him to be truthful on his application. I hope that he will be granted a licence despite his diagnosis; he is medicated and stable. If they won't give him a licence there will always be a gun for him to borrow from my collection. If it does come to that, I will stick to the letter of the law and always supervise him directly or, possibly, loan it to a mutual friend who is licenced (who will supervise him directly). Not because I agree with the idea that (stable, medicated) depressed people should be forbidden firearms, but because I'm not ready to be the martyr who tries to get it changed.