Legacy is the name of the company that runs the event series and it's not related to the constructed format which shares the same name. Legacy (the company) was founded in 2000, the famous constructed format would see its inception years later.
A Legacy Magic Showdown is an event that will take place 9 times during a 12 month period, each Legacy Magic Showdown will be held during 3 days. Friday will be dedicated to side events, you can expect a rich selection of formats and events, side events won't stop the following two days but the spotlight will be on the 2 main events. Saturday, the Grand Open Qualifier will start, a GOQ will last 2 days, is an open event which means that players can buy tickets until full venue capacity is reached and registrations closed, number of tickets available for GOQs will vary based on the venue. A GOQ is a separate event from the Classic Qualifier and will qualify 32 players to the finals, the final part of the swiss rounds of the GOQ will be held during the third day. Sunday, the Classic Qualifier will start, a CQ will last 1 day, has a maximum number of 256 players and will qualify 8 players to the finals.
Yes, you can let someone else use your VIP Ticket Side Event Vouchers.
Last Chance Qualifier will take place the day before the Finals, will be competitive-level events with an open number of players, and will qualify the top 16 players to the finals
No, the Legacy Magic Showdowns and Legacy European Tour as a whole isn't connected at all to the Grands Prix of old.