Hi there,
Some useful searches for you: cancer and omega 3
Lots of information about the benefits of omega 3 including helping with chemo.
Cancer and mushrooms: There's a great TED talk by a guy called Paul Stamets about mushrooms and their impact on cancer.
My oncologist has advised me against any supplements (including mushroom supplements) until after chemo. She tells me that my chemo works by oxidising the cancer cells, so taking anti-oxidant supplements can counter act the chemo!
I think it's best to check anything with my doctor because sometimes we think we're doing something healthy and it turns out to be a really bad idea.
I also recommend googling breast cancer and alcohol. It turns out that just 400ml of wine a night can double our chances of breast cancer. It's a category 1 carcinogen (same as cigarettes) but nobody ever says to a cancer patient "Sure, it's fine to have a couple of cigarettes a night if it makes you feel better." The Cancer Council have a policy on putting the same kind of warnings on alcohol as the warnings on cigarettes. I don't know why this hasn't happened. It seems like common sense to let people know the risks.
Good luck with everything. I think you're right to focus on diet and everything else about what we put in and on our bodies.
Every little bit helps.