…Obviously you can’t complete it, but in the multiplayer version, is it legal in the first place?

A: Since there are often cases where floating islands have no Beacon because they do not yet have a Beacon, the extension of this possibility is that islands in excess of seven which already have Beacons should be able to exist as well, under some imaginary condition in which there might be further Beacons in some extended version of this world.

One might also simply imagine they are islands outside of the seven which have been set up for the grand “game show” we are playing.

As long as it’s possible for all seven Beacons to be played, it’s fine to have islands in excess of seven. Remember: floating island fragments will attach to each other if conditions are possible, so it’s actually quite unlikely to have more than seven islands occur until all seven Beacons have been played, and even then, it’s quite unlikely.

Should they occur, however, it’s true that any extra islands would not be able to be completed, since you must always leave open a place for a Beacon to be played, and there are none left to place. But the existence of extra islands would not cause a problem in the game.