The first thing is to get some drainage holes in it YESTERDAY. The rest is up to you as to what you want to do. The container might drain well and the soil might be usable. My suggestion would be to go to your favorite big box store or garden center and see if they carry pine bark mulch in 2 cu ft bags. That would be the beginning of a new soil mix you could make by adding a little peat (or possibly some of your MG mix) and perlite and lime to it, otherwise known as the 5:1:1 mix.

I think I am going to go with a lime. My grandma has a huge fig tree so I have plenty of figs. Blueberries are my least favorite berries. Never heard of a mulberry before. Some quick googling makes me think it will taste like a more tart blackberry?

I have a whiskey barrel with a really big Rodgersia in it. It's gorgeous! Has been in that big 'ol pot, for oh maybe 6-7 years now. I leave it on the patio all winter. It's on my lower patio, so I cannot drag it up the stairs, obviously. I push it against the wall of the house and barricade it with bags of leaves or straw bales (which I subsequently use in the spring as mulch). I've grown so many things in pots over the years - tree peony, roses, etc. - and have overwintered nursery pots of unplanted stuff I didn't get planted. when I moved years ago, you should have seen the amount of stuff I potted up in the fall, kept in the garage over winter, and then dragged to my new house in the very early spring - perennials, roses, even a couple Japanese maples. If I need to overwinter something in a pot, I drag what potted plants I can into the unheated garage to overwinter (this year, just a tree form hydrangrea, I think that's all). I have never lost any potted plants over the winter in the unheated garage, and they freeze solid in there. I just make sure they're well watered before dragging in there, and come about end of January of February I pack a couple inches of snow on each pot so when the spring thaw comes, the melting snow slowly and gently waters the pots. The barricading method works well, too, if you don't have garage or shed space for storage. The point is: Go for it! :0)

26" diameter is wonderful! I would be tempted to put 6 TB's in there if they were ones I wanted to save. If buying I would do 3-4. Yes, dwarf iris do take less space. Go with your heart. You will then be thrilled every time it blooms. Were your Grandmother's all the same? If different colors are they in separate locations for you to get a piece of each? How many colors? etc. etc. If the soil is diggable, you can dig up pieces like those marked with a circle. Now. The 2 X's are dead mother rhizomes. This clump has a bad growing pattern. Some do, some don't. In need of serious clean-up. One piece is enuf to start a clump. That piece will be broken off its mother and should be left overnight for the cut area to dry (callus). A raw cut put into soil immediately is vulnerable to rot. The rhizome will look like a small potato. Leaves may not be visable yet in your zone? This link has much better pictures and explanations. Here is a link that might be useful: http://www.schreinersgardens.com/how-to-grow-care-for-bearded-iris