Snowball Earth brightness
Earth is already pretty bright due to cloud cover, with a typical albedo of .3-.35 -- that is, it reflects about a third of the visible light that hits it.

That means it couldn't get more than about three times as bright even if it were perfectly reflective (albedo 1.0), which means about 1.2 magnitudes brighter.

Spotting Earth from a great distance with the naked eye is less an issue of brightness, more an issue of separation from the Sun. I can't run numbers at the moment to get actual magnitudes or angular separations.