“[working backwards,] you must inevitably get to a time when no physical laws existed.”
Not necessarily. The laws may exist, the issue is when there is nothing for those laws to apply to. The law of gravity still exists, even if there is no mass for it to act on.
You can’t use “something” to work back to a time when there was “nothing”. An empty universe with physical laws isn’t empty at all.
If gravity didn't exist then how could there be a law 'governing' it? I mean, mass has gravity. It's not an outside magical force that acts upon mass. No mass, no gravity.
Generally, a law is a provable dependency between observable entities. I'm not sure what would the law look like if there is neither space or time for it to exist in, nor objects to apply to.