All topical Finasteride preparations tested (even those made with advanced formulations) have been shown to go systemic in studies. You might as well be taking the oral version. As someone who has been dealing with Post-Finasteride syndrome for the past 4 years, I do not recommend that you touch this drug in any way, shape or form.
Regarding allopregnanolone, since Finasteride is not supposed to inhibit 5-AR1, it should not do that, at least not directly. The problem has mainly been observed in people who have persisting side effects after quitting, where there is likely a negative-feedback event that occurs after the sharp return of DHT which essentially drastically downregulates the 5-AR1 in the CNS. This is then the cause of persistent brain-fog and a host of other neurological issues. I have them, and believe me, you do not want them.
Have you tried 5a-dihydroprogesterone to skip the 5ar-dependent step of allopregnanolone synthesis? Might help with the brain fog.