Respectfully, there is no way to predict whether you are going to charged. I suggest that since you do have some exposure based on your facts, that you consult directly with a criminal defense lawyer admitted to practice law in CA before anything does occur, so you will understand the potential consequences of the so-called investigation. When you are having the consultation, the lawyer can explain to you the reason why this is a theft-related charge, and not a burglary charge. Good luck.
This answer is provided solely for informational purposes, for you to use as a starting point when speaking directly with a lawyer in your State. I do not practice law in your State, and this answer does not constitute legal advice, create an attorney-client relationship, or constitute attorney advertising. I urge you to immediately contact an experienced criminal defense lawyer admitted to practice law in your State before you make any decisions about this case.
Based on what you wrote, yes, you could be charged with petty theft and second degree burglary for your actions.
You (and I) don't know the entire extent of the investigation that is or was ongoing, but from the comments the officer made, it seems like they cannot identify you as the person who did the fraudulent return. Either that or they were just trying to get a statement from you in light of all the evidence they already had.
The statute of limitations on a misdemeanor is one year - you could face charges any time within that time frame for the misdemeanor. Technically the second degree burglary (entering with the intent to commit a theft - in your case via the fraudulent return) is a "wobbler" in California and can be charged as either a felony or a misdemeanor. A felony has a three year statute of limitations.
I'd suggest you stop talking about this and hope & pray they don't file charges.
A commercial burglary is a charge that can be either a misdemeanor, punishable by up to one year in jail, or a felony, punishable by up to three years in prison. Commercial burglary is entering a business with the intent to commit a crime, such as theft or fraud. You may be charged with this crime, or with petty theft.
If you provide false information to a police officer, you could also be charged with making a false police report. This could occur if you lead the police to believe that you are an identity theft victim when you are not.
I'd suggest hiring a criminal defense attorney, who can assist you in handling the matter from here.