Mental Illness in a Natal Chart
I've suffered from major depression multiple times in my life and my Mercury is in pretty good shape. It opposes Jupiter and conjuncts Venus. I think it's due to my moon that conjuncts Uranus, squares sun and opposes Mars. But I do also have that nasty 12th house Saturn squaring Pluto in the 4th.
My husband also suffers from anxiety and depression and his Mercury is unafflicted. He has a Capricorn moon in the 29th degree conjunct Mars and squaring Uranus. He also has a 12th house Pisces sun squaring Neptune and a 4th house Saturn. Poor kid was adopted and never fit in with his family. Interesting how literal the chart can manifest.
Along the mercury vs moon lines, neither my husband nor I tended to think negatively, it was more of a negative feeling deep inside. Which makes me lean towards the moon.
My son has an afflicted Mercury and it has shown up as learning disabilities like adhd (also a mental illness I guess) and writing problems. He is a very cynical thinker, but his moods seem fine. His Mercury conjuncts Uranus and squares his 12th house Saturn. He's also pretty brilliant despite the afflictions.
Moon-Pluto may signal intense emotions. At worst, perhaps a mood disorder?