This you had Cushing here, one of the worst heretics in the hierarchy, slapped down even by the Vatican for claiming that Jackie was "free to marry" whomever she pleased.
https://www.thecrimson.com/article/1968/10/26/furor-over-defense-of-jacqueline-leads/
All right, I'm kind of lost here, she was widowed, wasn't she, in fact, free to remarry whomever she pleased --- as long as he were a man free to marry in the Catholic Church (or, in the case of someone non-Catholic, "free to marry a Catholic")? Isn't the latter part kind of implicit?
I don't know whether Aristotle Onassis was considered as already being in a valid marriage or not. He married a Greek woman (Tina Livanos), presumably in the Greek Orthodox Church, so it would seem to be valid.
Which raises another question: if two Orthodox schismatics marry before someone other than an Orthodox priest, does the Catholic Church consider their marriage valid?