re: Ever try to exit a store through a marked exit and it was locked?

You are going to catch hell here, but good for you.
Many of the worst fires involving fatalities were the result of locked exit doors. Once the fire occurs, it’s too late to take those things back.
In this case you did something before someone was hurt or worse.
OSHA fines are going to heavily depend on whether that particular location has been previously warned and fined in the past. They will likely not be severe if it is the first time. But if OSHA has been there before and the actions are deemed willful, they will greatly increase the fine.
The other problem may be the size of the company. Unfortunately, many large companies get fines reduced or removed that wouldn’t happen for a smaller company.