As far as I know, the lawyer's secretary is right. The reason is that your application for FM3 superceded your FMT, and it's no longer valid, regardless of the expiry date on it. The practical question, though, is whether you would get caught, and I honestly don't know what the chances are.

If I was in your position, I would just go ahead and get the Permiso. It's not hard to do nor expensive, and it doesn't affect your FM3 process at all. Then you could travel with the assurance that you will be allowed to leave the country.