Hi Mary - I'm so sorry to hear your husband has been ill. Bless you both.
You don't have to have flowers to feed your swallowtail. You can try putting sugar water on a sponge for it. Here's a paragraph on hand feeding from the FAQ.

"HAND FEEDING - Usually feeding isn't necessary, but in cases of extended bad weather there may be a need to feed your unreleased butterflies. In the wild, lack of sunshine and/or cool temperatures will make butterflies inactive, reducing their need for feeding. There has been quite a bit of discussion on what sugar/water ratio is appropriate for butterflies. It has been argued that butterflies require less sugar than there is in commercial hummingbird nectar mix, but it has also been argued that hummingbirds and butterflies feed from the same nectar sources in the wild. The most common formula seems to be 1 part sugar to 4 parts water. One tablespoon of sugar mixed with four tablespoons of water is a typical batch. Gatorade has also been recommended. Soak a small sponge with the liquid or place a few drops on a tabletop. Avoid large puddles; you don't want the butterfly's wings to get wet and sticky. Hold the butterfly by the upper wing tips and unroll the feeding tube with a toothpick, placing the tip in the mix.
Butterflies are surprisingly cooperative once they get a taste and will often sit and drink unrestrained until they've had their fill."

I don't know about keeping it in your greenhouse. Maybe someone else can comment. It would need to warm up its wings to be able to fly.

Best of luck.