Dry rootballs are often the result of textural differences between the soilless potting medium or claylike field soil the plants came from the nursery in and the soil on the new site. If the rest of the bed was moist with the heucheras only being dry then it was something like that - something involving the heucheras specifically - and not the overall conditions. This should have been apparent when these were dug and pulled apart(?) but heucheras specifically are prone to weevil larvae gnawing the roots off and leaving a sort of nearly rootless husk sitting on top of the ground, unable to keep itself turgid without diligent watering - and maybe not even then. The same thing is common here with primroses.

After a year hiatus on this project, I am resuming planning and hope to install this fall. I am hosting a pair of community collaborative design workshops to get input and teach the neighborhood about rain gardens & stormwater management. Last fall, non-profit group planted river birch in the area, with the plan to design the garden around the tree. The City also cleaned out the catch basin, the results were underwhelming...

Is this all around a no-go? No. Go for it. The concepts, principles and many of the plants that you will learn in a Rain Gardener class can be applied to planting this space appropriately even if it not used to manage storm water or as a rain garden. Take the class. Plant this garden after a good assessment of the site conditions, the markets needs and the actual human traffic patterns. Both will be good for your professional development.

Sounds good! Looking forward to seeing your rain garden plans!