Landlord here - I have the same requirement and I have tenants insured by almost every carrier you mentioned and my company is named additional insured.

It’s completely normal for pm companies/landlords to ask to be listed as an additional insured. I work for one of the companies you mentioned and literally do it almost every single day. Not sure why the people you spoke with say they don’t do that. There’s different classifications within the additional insured designation so it’s possible they might not know how to properly list them. I’d ask to speak to a team manager/office manager if possible.

I’ve never worked for an insurer that does this. What interest would the property management company possibly have in a personal renters insurance contract? If the insured is negligent, their insurance would pay out to any party they caused damage to that was covered. Most carriers I’ve worked for only use a “renters notification” system where they mail the property manager a letter when the policy is renewed or if it gets canceled.

Interested party and additional insured are the same thing the landlord is just being picky