Are there any measures that can be taken to help the ivy recover? Will a thorough spraying of water several days in a row help?

You may try some foliar misting to remove any residue to prevent further damage, but once the damage is done, it's done. It should only be foliar, through phytotoxicity, so I don't think you'll lose the ivy. But, you cannot take a damaged leaf and make it look good again. You may have to eventually prune it and let it rejuvenate or the damage can evolve into other opportunistic foliar diseases what grow on damaged tissue.

However, since the plants are huge - 7 or more years old - I'm starting to feel that the damage will, as you said, be only leaf damage. I am blasting them a couple of times a day with water (this covers the whole back of the house - a LOT of ivy) and am trying to get rid of any of the Sevin that remains. Boston Ivy leafs back out quickly, if I can get all the residue off the branches...

But in any case, it's deciduous and will drop everything after fall and start fresh in the spring. So even if it looks rotten the rest of the summer, it will probably just come back normally. I hope so, anyway!

On the up side - maybe I finally killed all the pests it had, ha!

This is a good lesson for everyone to learn. The labels of insecticides have a list of plants that can be damaged by their use (if applicable). That label should be reviewed each and every time a product is used. By doing so, we don't have to 'learn the hard way'.