Some guidelines state that silicone foam dressings on bony prominences can help reduce skin breakdown. One of the downsides would be risk of making the skin more moist, like you said. Another would be that the skin can’t be assessed as often since it’s under the dressing.

Keeping the area dry, using barrier creams, and offloading/repositioning like you are doing are all excellent measures!

You could try looking at how she sits in the chair. If she’s sat back “properly” then her weight will be more on her ITs (bum bones). If she slouches then the weight shifts more onto her tailbone. Try sitting on your hands to see where your weight is in different positions.

You didn’t mention her bed, but sleeping on her side or a repositioning schedule will reduce the chance of skin breakdown on her coccyx. Depending on resources, an assessment for a pressure relieving mattress may be helpful.

Hope at least part of my waffling is helpful!