I think you summed up your choices pretty well.

You could just enjoy it for now in the house and after it's done blooming let it go dormant for the winter. Although an unheated garage would be better than the basement. 40 to 50 degrees is not cold enough for it to go dormant and it will try to keep growing even without light. And if you do decide to let it go dormant remember to give it some water every month all winter.

Planting it right now might be a real shock for it since it's been growing all this time in a greenhouse. Minis are own root for the most part though so even if it dies back completely to the ground it could come back from the roots next spring.

I'm just a little warmer then you but I always buy mini's during February. Some in December. I enjoy them inside for a few days then slowly start moving them outside over a weeks time. They end up on the south side of the house burried under a pile of leafs. Have never lost one yet. I do move them into a little larger pot when I start moving them outside. I give them a drink of COLD water every couple of wks. No food at all during the winter. In the spring I strip any foliage that might be hanging on and prune the lightly. They come back like gang busters.