Wasn’t the studio that changed the ending, it was Howard Hawks. He never liked the idea of his characters getting killed at the end. As his career went on, he got more into the characters and less what the plot “should” have determined.
I like the ending of Red River because it defuses all the phoney hate between the two. It’s actually a very advanced narrative idea. By the time Hawks got to Rio Lobo, though, he’d completely lost interest in coherent storytelling. But as late as El Dorado, he was making terrific movies.
I won’t disagree with you about Howard Hawks being the one who changed the ending of Red River. I will disagree though about the use of the term “phoney hate”. Wayne’s character was single minded in his efforts to move the cattle and while that single mindedness CAN be a good leadership quality it was obvious that the character was, to say the least, a little lacking in people skills. As I said earlier, he was an a**hole.
In addition you had very real animosity between the actors. John Wayne found out that Montgomery Clift was a homosexual and Wayne treated him like...well...like you can expect a 1940 John Wayne would have treated a homosexual. The bottom line is that the last scene where they had to act like they loved each other was the only scene that required acting.