How do you properly install canvas keys?
Canvas keys work by expanding the mitre joints at the corners of stretcher bars (see sketch below). As the canvas keys are driven into their recessed slots on the stretcher bars the canvas, which is fastened to the stretcher bars, slowly tightens.

This action "depends" on the mitre joint of the stretcher bars not having been nailed or stapled together.

If the joint is glued, nailed, stapled or rigidly fastened then installing canvas keys will not be able to do their job. Actually, as you point out, driving canvas keys into stretcher bars that have been nailed will break the bars.

Note, once you nail the mitre joint closed you no longer have a stretch, you now have what is called a canvas strainer.

Large painting canvases, ones with cross members, are more the strainer then stretcher variety. 

The benefit of a strainer is you have a large ridge frame onto which to fasten your canvas. Large canvases shrink quite a bit as you work with them. So a rigid strong support is required.