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The tree will look unsightly for awhile, but it will regrow its leaves with no extra care as long as the stems and branches are still green. You should see new leaf buds develop where the old leaves were in about 1-2 weeks.

Mid to late spring is best, but brugs can take pruning just about any time. This article will help: https://www.gardeningknowhow.com/ornamental/trees/brugmansia/prune-brugmansia-trees.htm

I would leave them for now and prune them in the early spring (pruning is best done in either late fall or early spring). This will result in a bushy and full plant in the summer with lots of blooms. In the future, you may want to consider letting your brug go dormant in the winter. They just don't look all that nice indoors as they really try to go into dormancy, so it easier just to allow them to do that. Here is information on doing that:
https://www.gardeningknowhow.com/ornamental/trees/brugmansia/brugmansia-winter-care-wintering-brugmansia-in-your-home.htm

The following articles should be of some help to you:
https://www.gardeningknowhow.com/ornamental/trees/brugmansia/growing-brugmansias-in-containers.htm
https://www.gardeningknowhow.com/ornamental/trees/brugmansia/tips-for-propagating-brugmansia.htm
https://www.gardeningknowhow.com/ornamental/trees/brugmansia/getting-your-brugmansia-to-bloom-and-produce-blossoms.htm

I would treat the plants with neem oil. It will kill both any bugs or fungus that attack the plant and it is safe for people, pets and beneficial bugs. Here is more information:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

If the lower stems are green and firm, then they will grow new branches from those stems. Just give them a little time. If the Y died, you will simply have a bush form instead of a tree form.