Did the Guardian attempt to take over or attack Earth? I never finished the game, I've never been much of a gamer outside of D+D and when MMORPGs came out (in particular, this one).
No, it was never mentioned, written, or implied that the Guardian even attepmted to enter Earth. Only his verbal threats to the Avatar were as close as he came.
I think that was the Guardian's ultimate power - to corrupt, torture, or taunt with people's minds.
Yet Minax succeeded in destroying Earth, in it's year 2111.
There's several interesting things here. Let me quote the
Ultima Wikia here to start....
The Player, again as the Stranger, doesn't have the task to save Sosaria, but instead Earth itself from the Enchantress Minax. Being the lover and apprentice of Mondain, she is quite angry over his death by the Stranger's hand, and swears revenge. Manipulating the timeline to this end, she let Earth die in an atomic holocaust in 2111 - all life on Earth perished in the Aftermath.
The Stranger, having escaped from the changes in the timeline at the last moment, has to decypher the mystery of the Time Doors, which enable time travel, to reach Minax and prevent these horrible events from ever happening. Gathering the only weapon that can kill Minax, the Quicksword Enilno, and wearing the protection of the Force Field Ring, the Stranger travels back to the Time of Legends and confronts Minax in her castle to kill her. With her death, the timeline returns to normal, with no one remembering the horrible events that occurred in the changed timeline - all except the Stranger.
So Earth has always been in play, more than just as the home world of the Stranger/Avatar, British, and a few others.
The Time of Legends is a very strange thing. It's a place, a land, where Mondain's castle stood (and is seen in the intro movie), as well as Minax's castle. It's also been said that the Time Lord can freely go there. Nothing I know of has ever been learned on how Mondain got there, or anything else. It's a place from where both Sosaria (and all it's shard versions, assuming Minax still lives there), and Earth can be traveled. It's been called "Outside of Time). All told, it's like a myth that exists, but outside of reality. Much like the Land of Oz (which we know Garriott used for some inspiration).
The Qicksword Enilno, it's been noted that spelled backwards is "Online". And U2 came out in a time when online gaming was just getting started. No surprises there, but a point of interest. This reference to online gaming comes out in an Ultima game that includes not only a hero (the player) from Earth, but with Earth itself being brought into the game. I think it's clear that Garriott was looking to the future, to UO, as he made everything from U2 on.