"Could all these illnesses including depression converge to a
single cause?"
Possibly, yes. But, given what you have posted, it won't be easy
to find. (See http://sankey.ws/mcs.html for some of my background
on this. Note: I'm not a medical doctor.)
"To be very honest, I am sick and tired of taking medicines /
supplements. Can I tune my diet in a way that I don't have to
take any supplement?"
Supplements, yes. See http://sankey.ws/dietpills.html for some
ideas. But, I strongly recommend against discontinuing
medications given what you have posted.
"Also, what is the best form of eating a fish? I take fish fried
in omega - 6 oil i.e vegetable / canola oil."
I poach them. Water kept a bit below boiling point. Cook until
the fish is just barely milky all the way through, then get it
out before it loses any more nutrients or taste. Look for fresh
or frozen cold-salt-water fish - herring is widely available
world-wide. Fish canning processes remove a lot of nutrients.