Yes, if your batteries get to zero (which is not actually zero) your home should just pull power from the grid.
Some installs have the ability to do that, but it usually requires a tesla inverter / powerwall + setup from what I understand. Failing that, I think the short answer is "you dont, at least not without disconnecting yourself from the grid and running just off your solar and powerwalls, if you have enough energy to do that".
Note that I am just another regular person on the internet (although I do read most threads in this subforum, and this subforum has some amazingly well informed and smart people. I am always amazed at what I read here in this corner of TMC).
You did mention that your solar installers "were the experts" so they should be able to tell you if this is possible with how they set you up. As for operating before PTO, You say "I left it how they set it up, they are the experts". This is kind of like your taxes. You may have a tax preparer do your taxes, but you are still responsible for them.
Same deal here. If you violate the terms of what you agreed to (which is not to export anything prior to PTO) and for some reason get zapped by PGE, it will not be your installers issue. It will be yours. Its unlikely, but not impossible. Check your application for permission to operate to see what you agreed to, then make your decisions on whether you will "just let it run" after that.