You cannot use these photographs because you do not own the copyrights in them. Further, your former employer may not have had permission from its clients to allow it---or you in your capacity as competitor---to use these photographs. Without permission from your former employer and its clients, you should not use these photographs.
There may be a way around this----if photographs of your work appear on web-sites of your former employer or its former clients, you may be able to provide links to those works. Although I am in the loyal opposition on this, most people believe that such linking does not constitute copyright infringement.
Also, you certainly should be permitted to identify the clients of your former employer for whom you performed work. Indeed, depending on your relationships with them, you probably could safely use them as references. What you cannot do, however, is show these photographs without permission.
Probably not. Under copyright law, an employer is automatically the owner of any copyrights produced by its employees in the course of employment. This is the so-called "work for hire" doctrine.
Your question about essentially giving attribution to your former employer is a too-common misconception. Copyright infringement does not cease to be copyright infringement just because you tell the world whose copyright you are infringing.
Your absolute safest bet is to just walk away from all the photographs you took while employed. It may sound harsh, but it's true. That said, there is nothing improper about telling your prospective customers that they can look at your former employer's stuff for examples of your work. Nothing prevents you from rightfully claiming to have been the person who did the work. You just can't claim the work as your own.
Answers and information provided here does not create an attorney/client relationship.
No, you cannot legally do this without the permission of the company you were working for at the time you get these projects and took those photos. If you are an employee of that company, these were "work for hire" items and the copyright belongs to the company not you. If you were a contractor rather than an employee, the situation might be different, but that is not what you said. Since the owner of the company is upset at you for competing with them, you are taking a serious risk that you might be charged with copyright infringement for using the company's copyright protected photos.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.