Leyline of Combustion's triggered ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

If a spell targets you and/or a permanent you control multiple times, or if a spell targets you and one or more permanents you control, Leyline of Combustion's triggered ability triggers once.

A player's "opening hand" is the hand of cards the player has after all players have taken mulligans. If players have any cards in hand that allow actions to be taken with them from a player's opening hand, the starting player takes all such actions first in any order, followed by each other player in turn order. Then the first turn begins.