Alpha decay is an event that takes place within the universe, against a background of fields, particles and laws. The beginning of the universe is an event which, by definition, has no background. But without a background and without laws, there can be no meaningful probabilities. Hence the example you cite is not a parallel case.
However, if it is true that laws of nature do not apply when “nothing” exists, then there is no law of conservation of mass/energy to prevent an entire universe coming from nothing, is there?
You are talking as if the only thing that’s real about the universe is the net quantity of mass/energy: if this is zero, you believe there is no problem. But what’s ultimately real, in quantum physics, are the fields themselves, each of which has its own distinct character or “nature,” with its own well-defined properties. In other words, quantum physics, like chemistry, is deeply essentialist. No essences, no probabilities, and hence, no likelihood of a universe.
If the statement “laws are incapable of acting” is sufficient to dismiss an argument in which the universe is proposed to come into being thru processes that do not involve a god, then it should also refute those arguments in favour of God that rely on scientific laws.
I agree with you that laws do not act. (And if God were merely an abstraction, as the laws of nature are, then God would not be capable of acting either.) The “law of causation” doesn’t do anything; neither does the law of conservation of mass/energy, which merely reflects an underlying symmetry in the cosmos, according to Noether’s First Theorem.