Wyrds giving good luck was on my mind when I was puzzling out the project, but got set aside because a *lot* of the systems Witchery might be applied to are very light, low-numbers stuff, which break very very easily under heavy bonus stacking - and being able to bless and curse, both, means that such stacking would get *really* accessible.
I figured I'd come back to it and try and solve that, but never came up with anything satisfying.
I am not surprised that you had already thought about that. I do not disagree with you about the concept of bonus stacking can break things. However I feel that it is all about scale. It could be that s good luck charm or such works for a month, but only negates one danger after rolling once, unless it is very very specific. Trading power of the blessing to wider applicability and vice versa.
It was great to hear your thoughts however, I see your point of view.