I used to work for accounts receiving in a company. My boss(then) and colleagues in that area are no longer with that company. That area's work went to another department in the company and the area's employees left. Before leaving, I got a recommendation letter from my boss(who is now retired) on company letterhead stating I worked there for 3 years, using different accounting software. At my current employer(a university), they want to file a green card for me and asked for my past experience letters.

1. Can I use the past experience letters(for green card process) considering the department(where I used to work) in the company does not exist now, former boss(retired now) and colleagues are no longer with company?

2. If yes, what areas can CIS question the application? The issue is HR of my past company can confirm dates I worked(X to Y), but would not know particular skills/tools I used so if CIS wants to verify I used accounting tool A for three years(which the reference letter would mention), it might be difficult.

3. If I cannot use the past experience(reference) letter, what are my options to claim that experience? I do not know where my colleagues went. Last, I checked(before two years), my former boss after retiring had embarked on a world tour.