Here is my interpretation:
Scenario 1 -
GridlockYou would pay U. There is no need to pay more mana into each target so you would need to pay 1 for each target, which is in turn reduced by 1 for each target, making it 0 for each target.
Scenario 2 -
Rock SlideIt would depend how much damage you wanted to do.
If you wanted to do 1 damage to each, it would cost R (10 damage divided by 10 targets. The 10 for X is reduced by the number of targets so works out at 0).
If you wanted to do 2 damage to each, it would cost 10R (20 damage across 10 targets. So X is 20, minus 10 for the number of targets).
Basically you decide what value you want X to be, take away the number of targets you want to target, add an R and that's your cost. This formula is important as you don't have to do the same amount of damage to each target.
Consider a board with a 1/1, 2/2 and a 5/5 you want to kill. You need 8 damage total and there are 3 targets. 8-3+R. You pay 5R.
Scenario 3 -
Even the score.
You pay for this as normal as it doesn't target, so gets no cost reduction from Hinata. You want three cards, you pay three mana.
Hinata only cares about targets, not X spells in general, so no cost reduction for
Even the Score.
Conflagrate (or any XX spell) is interesting. Because of the way cost reduction is applied, you don't get to double up on your discount.
Taken from
Conflagrate rulings:
"To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you’re paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was."
Given the same board state described before, you want X to be 8 (so base cost is 88R or 16R). Then you apply the reduction of 3 for the three targets. You end up paying 13R.
Hope this helps!