#tailsfandango

In the second combat damage step, damage is assigned and dealt by creatures with double strike and by creatures that didn’t assign or deal their combat damage in the first combat damage step. A creature that gains first strike after the first combat damage step still falls into that second category and will deal its combat damage at the same time as creatures without first strike.

This is absolutely fine. Just discuss your situation with the head judge before the event starts so that they can inform the judge staff about what accommodations you’ll be receiving, and always call a judge when you need to leave your table.

The typical way that LED is used in Legacy decks is as follows: First you cast Infernal Tutor with other mana, then, holding priority, you activate LED for mana and discard your hand. When Infernal Tutor resolves, you now have no cards in hand and can search for any card you want, and you’ll have the LED mana to cast that card.

You’re paying the flashback cost, which is 3UU. You can’t pay two different alternate costs, and since you can’t cast the spell without flashback, you can only use that cost to cast it.