I would wait to plant in MI for a bit yet. I would also watch the tuber to make sure it has some viable eye. If so, planting at a 4 inch depth covering with 2 inches of soil and filling in as it grows should work well- just protect it from frosty nights, and slugs if you have them.

"loose clump connected by strings" is a total mystery to me, could be a small clump of tubers surrounded by old roots? Do you see any signs of growth near the old stems?

There are alot of good dahlia growers out there who back up their product, but perhaps you got a good deal. I hope they work out for you.