Soil can't be too terribly heavy and wet if arbutus live well in it. Is there any place to insert new shrubs without chopping through the sod-like roots of the arbutus? That would be tough going and the arbutus would not be likely to appreciate it.

Foliage on the ends with lower and interior sections opening up to reveal the bark is typical of arbutus. Maybe what you need to do instead of trying to fill in with additional planting is cutting occasional stems back periodically to induce new growth lower down. Be sure to read up on pruning of arbutus before doing anything major. Look at public libraries for books on pruning, in addition to searching the internet. It is usual for books on pruning and training of plants to have chapters detailing handling of specific plant genera, in your case it would be Arbutus.

A couple good ones are Brown/Kirkham, The Pruning of Trees, Shrubs and Conifers and Brickell/Joyce, The American Horticultural Society - Pruning & Training