You can't make the trail safe in all conceivable conditions, it's simply not possible. The idea is that one cairn will be visible from the previous one in all but the worst possible conditions. In those cases, groups need to know techniques to safely travel above treeline. That may include moving away from the current cairn until it's barely visible, then sending another person out farther but within sight of the first person, until the next cairn in spotted. You may have to use a rope. It's slow laborious work but a well prepared hiker should keep an eye on the weather and make appropriate choices.

Blazing above treeline is only moderately useful. If visibility is that low, blazes are hard to pick out. Once the freezing rain and rime ice starts forming blazes become almost impossible to spot. They also don't last long in that harsh environment and need to be repainted frequently.

As to improving cairn building, unless you want to make all trails above treeline into sidewalks some degree of difficultly is inevitable. Depending on the terrain and amount of use the trails get, I would imagine that cairns should be 50'-100' apart.