Your tomato/tobacco hornworm will eat the leaves of tomato, preferrably, though they can be found on other members of the Solanaceae family such as pepper and potato, though grudgingly. There are a few native weeds in this plant family, but you would have to look them up since I don't know where you live. Most of those plants are considered toxic....Solanaceae is the Nightshade family. That's pretty much it. They are not generalists but very host specific.

The lids of water are useless. Spray the leaves occasionally with water. Captive caterpillars often succumb to disease if in the hands of someone who has not learned the rudiments of caring for them. That would be most of us!

To go along with your philosophy, releasing them into the 'wild' without an appropriate food source would be cruel. They would suffer a slow death of starvation. I suggest that you either sacrifice your own tomato plant, or get some more tomato plants, or serve them up to the birds in your garden by placing them in the bird bath or feeder. That would be a good thing to do, right?

In nature, there are many pests and parasites that help control these particular caterpillars. Very few of the many that are hatched actually make it to adulthood.

Moving these Tomato Hornworms to a place birds will eat them would be one method of keeping them from devouring your tomaot plants. You might also have Braconid Wasps around that might parasitize them.

Here is a link that might be useful: Tomato Hornworms