1. What meaning is there in doing anything (being a doctor or a psychologist for instance... or any number of other professions) if we can't even trust the research or the schooling? How can I make a difference in the world or do anything useful with no real knowledge?

That makes things sound worse than they are. I disagree that we have no real knowledge, and I'm also not sure about lumping doctors or psychologists together in this context. In medicine there are effects so huge that explaining them away as publication bias or spurious correlations is implausible (maybe because the relative risk is so huge, as with smoking causing lung cancer, or because the base rate is so low, as with asbestos causing malignant mesothelioma), so I count them as real knowledge. But I don't know of similarly huge effects in psychology, so psychology might differ in that key respect.

(Here's a speculative tangent that belongs in brackets. The foregoing might partly explain bad epistemic habits in research. Historically, lots of research went into things we basically fixed with magic bullets. So it didn't much matter when people suppressed negative results or leaned heavily on observational studies; the true effect of the magic bullets was so huge that it held up despite the biases. This might've gotten researchers into the habit of not worrying about, or not finding out about, methodological biases. But now we're searching for smaller effects those biases matter.)

Better still, most of the problems you refer to above are solvable. We could, for instance,

• publish negative results
• learn about warning signs that can indicate flaky study results
• force researchers to publicly announce trials and their endpoint measures before their trials begin
• force researchers to disclose funding sources and possible conflicts of interest
• put more effort into searching the grey literature and foreign literature when reviewing studies
• focus on randomized experiments (and use placebo controls where applicable in medical or psychological trials) over observational studies
• impress the importance of evidence-based methods & treatments on practitioners and professional organizations
• use statistical tests for detecting publication biases in reviews and meta-analyses

So, supposing I did accept the premise that the research base is so bad as to make doctors and psychologists useless, there'd still be an obvious alternative to giving up and walking away: I could become an epidemiologist or a medical statistician or a policy pundit, and encourage people to do the things I listed above.