Q.How to care for/repair a mature cedar that was skirted
The cedar is mature, about 56″ around its lower trunk. It was skirted without permission. Will the branches grow back? This is in an urban area. It is part of a boundary that has rhodies and azaleas further along the line. Would these grow under the cedar now that it has been skirted? How much disturbance can the roots tolerate? Can holes be dug for a hedge, inside the dripline? Can soil be added as a top layer so that annuals and perennials can be planted beneath the cedar, inside the dripline? How close to the trunk should soil be added? Thank you!

Acid loving plants, shrubs, perennials and ground covers can be grown under the Cedar Tree.
Yes, to both azalea and rhododendrons.
Wintergreen and Japanese Spurge are good ground covers.
Big Blue lilyturf and Bergenia are good perennial choices, and Lady Fern and Christmas Fern.
https://www.gardeningknowhow.com/ornamental/trees/cedar/growing-cedar-trees.htm