4) How can minimize the collection of personal information by mixers?
Use Tor Browser. But if you're willing to use terminal with Tor/VPN with some trial and error, you can make POST request so the mixer have no information about your browser/OS.
5) What other weaknesses and vulnerabilities are there in the scheme "your address 1 - mixer - your address 2"?
Under the hood, usually it's more complex than "Your Address 1 - Mixer Address - Your Address 2". But the weakness heavily depends on how mixer mix user coin and user behavior. You might want to check
Breaking Mixing Services.