It may not have been the heat wave. It may have been damage during winter. When the plant leafs out and the leaf opens up, the damage then causes problems and the leaf get aborted.

If the plant was having issues with the heat wave, like needing more moisture, the leaf would turn brown from the edges inwards.

I would just cut the affected leaves off by cutting the petiole string that connects the leaf to the stem.

I am assuming that there are no pests, snails, etc around though.

I would not worry about delays in blooming, especially if the shrub was recently planted (last 1-3 years) as it takes about that long for it to become established in your garden. My paniculatas have bloomed early in April ro thereabouts but not this year so just wait a little longer. The exact date can be a "moving target" from one year to the next.

As long as the soil is kept mulched and is evenly moist and the downspout does not make water collect in the hydrangeas' locations, things should be fine.

The plants typically have "back up" invisible flower buds but you will just have to wait to see what the affected stem will do (bloom or not). It develops invisible flower buds from late Spring to Summer so give it TLC and see how it fares with bloomage.

UPDATE: They bloomed! Thanks for the advice.

All right! Good for those babies! Make a note in a wall calendar noting when they bloomed and pass the info on to the 2018 calendar so you will now when to expect 2018 blooms. More or less.