I'm not really seeing it, sorry.

The OP asked us to imagine what an aligned, super intelligent artificial intelligence could give us in FDVR that it couldn't provide in the real world. In a world where such an entity exists, there wouldn't be any space for bad actors to operate anymore. Any attempt to interfere with other people's experiences would just be intercepted before they could ever become a threat.

Now if you want to imagine a scenario in which FDVR is somehow achieved before ASI, then sure. Cyber criminals, terrorists and bad actors could be a problem for BCIs, just like they're a problem for smartphones and PCs today. People who go to sketchy websites and download random .exes without thought put themselves at risk of viruses, but most people remain relatively safe.

Governments may try to pass restrictive laws, but thus far the western world has been pretty permissive towards questionable content in virtual worlds, and everybody else can just pirate the content they can't legally get. Also, I think the political class would have a motive to allow FDVR to flourish unchallenged. People happily living in their own heads would have less motivation to vote against the status quo.

The idea of people invading other people's virtual worlds for the sake of it just sounds cartoonishly evil. It's catastrophizing. People who want violence against other people will join virtual worlds that are designed for that express purpose, just like how people play competitive multiplayer games today. When they are tired of it, they will return to their own private worlds where the only real people they'll ever see are the people that they want to have access, if any.