I would have your same concerns if I found that the area that I was digging a hole for the camellia was full of tree roots. They would definitely compete with anything that you wanted to plant, including grass.

Spruce tress can have shallow roots in the top 2'. This would present problems with camellias, azaleas and other ericaceous plants. If you really want to try, experiment with 1-2 shrubs planted very far away from the trunk where they get some sun and afternoon shade & use some kind of barrier to block the spruce roots. By the way, if you can determine exactly what king of spruce tree you have, your local plant nurseries could tell you how wide they get... useful info when planting the camellias.

Tell tale signs that one can sometimes use... Have you already had difficulty growing other plants or grass under the tree? If so, that may be a hint of problems to come.

I would think that cutting a few roots, enough for just one bush of camellia or something else, will be less stressful on the tree than suddenly digging 20 holes around the tree.