It sounds like you have fried more than just M7 and the 5V regulator (if that's the one that you're referring to).
In my opinion that would indicate that you also fried the 3.3V regulator.
This would indicate in my opinion that the TTL-to-USB converter is also damaged.
Not knowing what exactly happened, the 328P might also be damaged and either is dead as well or it might be dying slowly (meaning it's unreliable).