Can you plant trillium bulbs in spring?
The best time to plant or divide a Trillium is when they are dormant, so this can start late summer into early fall.
I live in Michigan and need to plant some trillium in spring. Do I plant it now?
Yes, you can plant out in spring after danger of frost has passed.
what is the appropriate way to deal with spent trillium blossoms?
You can snip them from the plant if needed.
Leave the leaves to die down naturally.
should I cut back my trilliums this fall?
If the leaves are ratty looking, trim them if you wish. Left in place, the leaves will compost and add to soil fertility. There is no requirement here, it is personal preference that guides your decision.
We have had Trillium flowers in our garden for years. Unfortunately while trimming yesterday, they accidentally got cut off. Is there any chance of them coming back?
It is possible that they will, especially if this is an established bed. Although they are sensitive in nature, they can recover from damage from foraging animals, and being stepped on. This article will give you more information on the care of these: https://www.gardeningknowhow.com/ornamental/flowers/trillium/growing-trillium.htm