Yes they are grooming still. Check out the 'Hancock and north' thread. I don't know when trails close.
Trails close the 31st UNLESS they extend the season. Normally only with ideal conditions will they extend. Best bet would be to check the Michigan DNR site and see if they announce a extention otherwise landowners insurance and permission expires at midnight the 31st
This topic is discussed every year and I don't recall all the trail classifications north of Calumet as this thread is discussing, but the fact is trail 3 does not close on March 31. This is a multi use trail and is open year round as are a lot of other trails up there. There are some trails that use private easements and will close, but there is plenty of legal trail riding available after March 31st.