It wasn't clear- sorry. It sounded on the one hand like a rant, but on the other like a post inviting opinions.
This is perhaps more suitable to Am I Being Unreasonable (AIBU) than this section where people ask for help.
The point is that no one really knows about some things and science is always proving and disproving things.
Don't know if you watched Trust Me this week (BBC2) but two highly qualified eminent scientists discussed if meat was healthy- one said yes, the other said avoid altogether. So even the experts can't agree.
I don't know the exact details about tuna but assume this is the possible mercury content you are talking of?
I'd take issue with a few things you consider you are doing right: sunflower oil is not healthy. It's high in Omega 6 and most experts say that we have too much Omega 6 in our diets at the expense of Omega 3. Olive oil is healthier. Also, you may not be aware but last year there was new research which showed that saturated fat (in cheese and meat etc) was not responsible for heart disease. The new culprit is sugar.
Two biscuits a day may well be taking you near you daily allowance of 6 teaspoons if you eat any other type of processed food or drinks with sugar in them. Also, using sweetener instead of sugar still keeps your addiction to sugar alive- far better to stop altogether.
I'd say there are some scare stories around some foods (never heard of the microbes in milk) which are not based on serious research.
If you want to keep up to date, something like Google Scholar will throw up quality research papers if you put your question into the search engine.
It's all about balance. No one should feel they are denied a treat, or the very occasional sausage or bacon butty, but the evidence is there to show that some food eaten often or in large amounts can cause harm.