For your question about opting out: if you are not in the container that the site "always" opens in it will offer you a choice of opening it in your current container or in the default container for that site, at least it does for me.
What I would like is the option to assign a site to two different containers, so mail.google.com would not offer me to switch when in in Personal or in Work, but would give me the choice to open it in either when I'm in a default container.
What I would like is the option to assign a site to two different containers, so mail.google.com would not offer me to switch when in in Personal or in Work, but would give me the choice to open it in either when I'm in a default container.