TV-MA is a more severe rating than R. TV-MA is the equivalent of NC-17 in the US. Note that R and MA are used by two different groups. The MPAA (Motion Picture Association of America) uses a rating system that you can review here where R is the second most severe, coming up behind NC-17.

Yes, it is indeed as of July 2024. That year saw the new German Interstate Treaty on Gambling (ISTG) or Glcksspielstaatsvertrag (GlStV 2024) come into effect, which legalizes and regulates online gambling in Germany across all 16 states.

A prime example for illegal websites are pedophilia related sites. Everything else is public and open, as per the principle of law that everything that is not explicitly prohibited is allowed. There are no banned websites in and of themselves (though there are websites of which the owner gets sued for the content).