The spell cares about what mana was spent to pay its total cost, not just what mana was spent to pay the hybrid part of its cost.

The spell checks on resolution to see if any mana of the stated colors was spent to pay its cost. If so, it doesn't matter how much mana of that color was spent.

If the spell is copied, the copy will never have had mana of the stated color paid for it, no matter what colors were spent on the original spell.

If you spent both and to cast Dawnglow Infusion, you'll gain (X + X) life. An ability that triggers whenever you gain life will trigger just once.

Whether you choose to spend both and to cast Dawnglow Infusion has no bearing on the value of X. For example, say you decide that X is 4 and you'll spend on {G/W}. If you end up spending to pay the cost, X is still 4 and you've spent both and . You'll gain a total of 8 life.