Another aspect that seems odd is how easily they stormed into her dressing room. Especially in a place like Vegas where I'm sure they had security well figured out. Seems like a hired bodyguard would be in trouble for letting them in and would be not doing his job even if he was familiar with one of them. [was Diana Ross that easily accessible then?]

Again , not saying it didn't happen.
Maybe once backstage the bodyguard acts as a go-between and then asks Diana if she wants them to be let in