In zone 8, can Brugmansias be wrapped and left outside?
In your growing zone, it should okay to simply cut the plant back and apply a generous layer of mulch for winter protection. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/trees/brugmansia/brugmansia-winter-care-wintering-brugmansia-in-your-home.htm
I have acquired some brugmansia seeds. When is the best time to plant them?
Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/trees/brugmansia/tips-for-propagating-brugmansia.htm
This is my first year with a brugmansia and it has three stems about 3 1/2 feet tall with only leaves at the top of each stem. Should I cut back the stems now to encourage new growth closer to the base or wait until spring?
Personally, I would wait for spring. These articles will help with both pruning and winter care indoors: https://www.gardeningknowhow.com/ornamental/trees/brugmansia/prune-brugmansia-trees.htm
If you have 2 offers for a brugmansia plant, one in a 4.5 inch pot or one in a one gal pot, which is the wisest purchase?
Brugmansia should be planted in large containers, at least two feet in diameter, to allow the plant enough root foundation for it to branch out - they grow super fast. With this in mind, I think your decision boils down to this - which of the two brugmansia plants is healthier? Regardless of which plant you choose, it will need to be eventually repotted, with more immediacy for repotting given to the 4.5 inch pot because that pot is really small.
For guidelines on choosing healthy plants, please visit the following link:
https://www.gardeningknowhow.com/garden-how-to/shop/choosing-healthy-plants.htm
For more information on the care of brugmansias, please visit the following link:
https://www.gardeningknowhow.com/ornamental/trees/brugmansia/growing-brugmansias-in-containers.htm
My brugmansia grown from seed last year grew to 6 feet tall before a Y. Will it still flower lower down?
It may flower eventually provided the stalk has nodes (small bumps) along the length of the stem. Nodes are the part of the stem from which a leaf, branch, or aerial root grows.
For more information on brugmansia plants, please visit the following link:
https://www.gardeningknowhow.com/ornamental/trees/brugmansia/growing-brugmansias-in-containers.htm
Many of its leaves, especially on the lower part of the plant, are turning yellow, wilting, and drying up. What am I doing wrong?
Yellowing leaves is generally a watering issue.
Brugmansia need plenty of water. Twice a day watering may be needed as the warmer summer temperatures rise.
Make sure the pot is well draining.
Here is a great article with care requirements.
https://www.gardeningknowhow.com/ornamental/trees/brugmansia/growing-brugmansias-in-containers.htm
My brugmansia leaves are being eaten by what looks like ants or very small, very black spiders. Any advice on how to deal with this please?
Spider mites and aphids are among the most common pests of this plant--both of which can be taken care of by treating the plant with neem oil. Here is more information: https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm