You guys thinkg a rim will work in this set up ?
The way the setup is right now should work perfectly fine, maybe even better since this will allow for a majority of the plants to grow up.
How do I prepare the potted plant for the riparium, just wash out of the soil and plant in a planter? Should I trim the roots at all?
Assuming you have a planter for a riparium ready, I would rinse the soil off the roots. Make sure you're not pulling the roots of as to not damage the leaves or stem of the plant. Then, I'd get a pair of shears and trim down the roots. Place a bit of gravel/hydroton on the bottom, place the plant in, then fill the rest.
How to divide a peace lily?
Dividing peace lily is kinda tricky. I'd recommend getting a smaller plant locally and using that. You'd pretty much have to divide it by separating the plant but there may not be a small enough plant for use.