1. Like most playtime events in the past you can log in your other characters since it is account based (if we go by the usual playtime rules). and its not character based
"Tailed Wisp will be send to the MAILBOX of the highest level character on the server."
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that will most likely mean the highest level character in your account. (correct me if i'm wrong)
2. The time starts whether or not you are not yet level 100+ but will be then eligible for the rewards if your character is level 100 by the time the event ends.