What form of exercise is the best for beating depression? Can too much exercise make it worse? If so, how much is too much?

No, but if you don't want to exercise, but feel compulsion to do it, that becomes it's own stress problem. I exercised, starting at age 74, every day, for two years. I started to overcome back pain, on the advise of a neighbor who was a retired back surgeon.

What are DIY strategies or treatment for situational depression besides time -- short-term form of depression that occurs as the result of a traumatic event or change in a person's life?

I recommend either camping, traveling or a light daily exercise regiment that takes place outside if possible. Personally, i ride my bike to the gym. I get some sun! I stop in places i wouldn't consider stopping if i was driving my car. Like a pet shop, a furniture store, a particular restaurant, etc. I take omega-3 supplements