When I see potato beetles on a plant, I examine the leaves to see where they have laid eggs, because they always have. That's why putting the plant on top of the soil next to other healthy plants will tend to result in more potato beetles throughout the planting area. Instead, I pick off the eggy leaves if there are not too many. If there are, with some plants the eggs can be squashed, but with others the leaves are too tender and squashing the eggs means seriously injuring the leaves. Otherwise, the plant just has to be trashed, and I mean put in the trash or burned. Otherwise, they will reproduce and seek out other members of the nightshade family in your garden. The nightshades are my favorite family of plants and I grow various members of it for seeds, herbs, fruits, roots, just the hell of it, whatever. This is the first time I have had a problem with these beetles on my plants outside of years ago on some squashes on some other land.
I have not found that growing marigolds through my garden has any effect on anything other than it looks good.

IME, also, it is not the common French or African marigold that is meant but Tagetes tenuifolia, which has a different smell. I have seen these sold as "Gem" marigolds. Sometimes the seeds can be pricey, but some places sell them in bulk so they can be sprinkled around. This is the same one that some people in the south use as a thick border planting to keep root-knot nematodes out of plots they have solar-killed nematodes in. I have heard good things from people who have tried that.
I use wide beds for growing, although some things are grown around the edges of my city lot as well or even just sprinkled around in areas that get dappled sun, like my currants, gooseberries, roses, and woodland shrubs. In the past I have grown things in groups without a problem, like 18 tomatoes in a patch. I must make use of what sun I have in my shady lot. This year it happens I have grown a lot more things in very tiny groups, like four plants or two plants, and I have actually had more problems than in the past with bugs, but it has been a real weird year weatherwise--huge amounts of rain, strong hail that smashed a bunch of plants, super high temperatures, windstorms, the works. Such volatile weather is difficult to garden around.