To tell the truth, it is really nobodys business of why she was in the car with Lisa and Vernon. That is my opinion, if she wants to tell then I am for it and will listen to it.
Was there a relationship between the two? Perhaps. My opinion again, she was the best looking of the Sweet Inspirations. She have always said that she loved Elvis lot and also respected him and that they was really good friends. Their could be a relationship between the two as lovers or as a brother/sister, a sister that Elvis never had and could tell her anything and would kept private between the two. Only she and Elvis knows.
Now, does it need to be known? I would say yes and no. Yes, so as you said would bring a light to Elvis that shows he truely wasn't racist and that he truely love Myrna. But I would say No, because it is really no one business. Elvis kept his life private so why tell something that is really private as a relationship.
I have no problems with interracial relationships what so ever. Their is several beautiful black ladies out there - Alicia Keys and Beyonce to name a few.
So it is up to Myrna Smith to tell the world how she truely felt about Elvis either it was two lovers or a brother/sister relationship.