The adults overwinter under plant debris, which is why good sanitation, tillage etc are so useful in controlling them. They are pretty much immune to Sevin. Here is a link that might be useful: Squash Bug

At this point you can simply take seeds right out of the fruit and plant them, since the seed does not need to be prepared for storage. If you have fruits that grew on separate plants, take seeds from each for maximum genetic variance. Also pick the largest plumpest seeds.

I agree with Remy. Squashes fall into six different species, so you could grow six species all at the same time without concern of cross-pollination. In your case, you have to consider ANY plants within 1/2 mile radius minimum. Must be hard in the city--I live on 20 acres. The neighbors zucchini is C. pepo so should be safe even if they still had some blooms still on. Other C. pepo squash are acorn, crookneck, scallop, small striped and warted gourds, spaghetti and of course zucchini. C. moschata squash are butternut, cheese, golden cushaw and many more. My BEST suggestion is to check out the article at the link below and then if you don't already have a book called, "Seed to Seed" by Suzanne Ashworth --- GET IT! Best investment for seed saving I ever made. It lists all the different varieties for all kinds of vegetables, along with the info. to determine what you can safely grow side by side without cross-pollination issues. It also tells how to plant, grow, harvest and dry seeds. It is considered our seed saving club's bible. This year I grew C. maxima, pepo, moschata and mixta species side by side. No cross-pollinatin, no problems. The book is great for planning your garden and seed purchases each year. Be sure to check out the article I posted about saving pumpkin and other squash seeds. There are several other seed saving articles at my site there; onions, sweet potato slips, seed life guide, seed storage, etc. Link is below. Here is a link that might be useful: How to Save Pumpkin Seeds, Squash & Gourds

That's a good point about DE and bees. But for squash, with a few big flowers, the way to go is probably just wash off the flowers after coating the foliage. Of course, hand pollination is another strategy. I believe bees are more immune to DE than other bugs. First of all, they avoid it, and secondly, they have lots of hairs on their bodies that collect the dust, and they just shake it off. I once heard that DE was used in beehives to control hive beetles. Gee, I don't think DE on the ground is going to stop anyone. Be careful to use food-grade DE. Other kinds of DE can have chemical insecticides mixed in. It's really easy to coat a plant with DE. I learned this a few years ago. You get out your shop vac and set the hose up to blow. Then you wet down your plants with a hose. While the plants are wet, take a big handful of DE, and use the shop vac to blow it into a big white cloud around the plants. It will stick to the wet foliage everywhere. It will stay on the foliage until it rains. You can do a few cubic yards of foliage in just a few minutes.