Hi there- I'm really sorry you are going threw this, and you are right it's really serious. It may be that you simply aren't able to get the information that you are looking for. While Google may have it logged somewhere deep in their system, that doesn't mean that you will have access to getting the data without a court order. I'm not sure the legal implications of what you are describing, but it's certainly worth talking with the police or lawyer and see if they have any suggestions.
The only option I'm aware of is the Google Data Download/Takeout, which I believe is what you are describing earlier. There is an option for email there, so it may be worth looking into, but this is more to see what data Google has collected on you, nor necessarily an audit log, like you are looking for.
Anyway, this is slightly off topic and I'm sure you've already gone through this, but it's necessary to ask yourself what else does he have access to? If he's willing to do this, what else is he willing to do? Any and all important passwords and accounts need to changed/secured on a secured computer, preferably one that's reformatted.
Best of luck with this shitty situation and I hope it gets better for you.