I don't see any barrier to insuring the unit. You just get the vin number from the seller, then call your insurance company and give them the make, model, year, and number. You don't have to tell them any of the gory details; they don't care, as long as you pay the bill.
People buy and transport used trailers and 5th wheels with just a copy of the bill of sale. If stopped while there's no tag, the driver would show the bill of sale and a letter (or contract) from you showing he's authorized to transport it to Michigan. I really don't think any cop (except a hard-a**) would issue a ticket under the circumstances you describe; it's expected that a person will not tag a trailer until it reaches its new home state.
I'm currently in the process of buying a TT in Idaho. I or my buddy (or both together, not sure) will be driving to get it and bring it back to OK. I'm not the least bit concerned about a license plate; if the seller removes his before sale (and he probably will), I'm fine with that.
Heck, I have been towing my cargo trailer for years to camp out of state, and it has no tag because in my state a tag is not required; I carry a statement from Oklahoma saying it's not required just in case I get stopped, but in 8 years of towing to and from Michigan and Colorado I've never been pulled over.
Stop worrying and just do it!