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Normally the plant will not come back, though in some areas it has been known to reseed. Therefore, it is possible that you could get one popping up somewhere in the garden. You can also allow it to bolt (go to seed) and then collect some for new plants next season.

In addition, the plant can be grown indoors as well. Here is more information on that: https://www.gardeningknowhow.com/edible/herbs/cilantro/how-to-grow-cilantro-indoors.htm

Cilantro is actually quite tolerant of cold conditions. It can survive temperatures in the low to mid 20s. However, if a severe cold snap is expected and you are worried about your plants, you can simply cover them with a blanket or burlap for additional protection.

Sounds like you've done about all you can at this point. It is possible, though, that the seeds could have been lost in the fall or may have gotten buried deeper when putting the soil back in the pot (this would make it take longer to sprout, if at all). All you can really do now is wait and see or you may want to consider replating some additional seeds and thin down the sprouts as needed should the others pop up.

This article will help: https://www.gardeningknowhow.com/edible/herbs/cilantro/how-to-harvest-cilantro.htm

The flowers are not poisonous. They are edible and many people enjoy them. You can also roast the seeds and eat them as a snack food.