Are there any plants that will repell or kill Japanese Beetles?
I've heard that they don't like mint - but will it help keep them off of nearby plants that they do like?
Besides Google says they like marigolds and should go for those over other plants - backwards from most pests ..... while I have seen a few on my marigolds - my intruders seem to prefer zinnias and morning glories to anything else that I have this year (so far).
Should I let the zinnias go to seed so I can keep growing them as a sacrifice or is there something better to do with or for these pests?

I don't know how well they work against Japanese Beetles, but Rue (Ruta graveolens) is said to repel beetles. I've recently tried planting some near flowering cherries that have been attacked in the past, but I haven't gotten enough out yet, or for long enough, to report on how well it worked.

I saw Japanese beetles eating zinnias (at someone else's garden) earlier this summer... what they really went after at my house was the evening primrose!

Leaving a sacrifice plant isn't a bad idea... It was easy to collect the bugs off the evening primrose and feed them to the chickens...

Now... if the leaf foot bugs were so easily collected...