After repotting it two times and seeing it barely stagger along, I researched and realized it needed to be planted outside, and that it was not, in fact, an indoor plant. Derp, I'm learning!

However, if I plant now(ish), will there be enough time before the first fronts hit for it to establish itself, or should I save it in the pot until next spring? The place I'm considering is next to the house, which is brick, so it has the advantage of possibly being more insulated.

After it goes completely dormant you can winter protect it by either mounding up with soil around the plant or with some kind of mulch. Leaves will work fine. Since it's a mini you can pretty much bury it in leaves but put a stake there so you'll know where it is come spring. And if you get a lot of snow it's OK to pile it on. Snow is a very good insulator and will protect and hydrate the rose all winter. In the spring when the ground thaws and night time temps stay consistently above freezing (that's April here but may be different for you) you can start to uncover it gradually. Once you start to see some new growth uncover it completely. Prune off any dead wood and she should give you a summer of lovely blooms!