Former employer not responding to referencing request

I have a former employer who simply does not respond to requests for referencing. They operate an online ticketing system for employment verification requests that is just a black hole. You file tickets but never get a response. There is no ability to escalate as all requests are handled via this ticketing system. My last manager at this employer is deceased and I have no colleagues still employed at this company. I do have former colleagues from this company who have offered to serve as a reference if I need.

Is there anything I can do in lieu of a company reference that could suffice for an HR department? I have pay statements, a copy of a reference letter from when I was still employed there (I had one for a mortgage application) and also the former colleagues who can be peer references. Should I offer these things, or is that not enough for a formal reference?