I only watched the last hour so I can't comment on much of the show.

But yes, usually the opposite gender of the preceding year's winners present the awards. So for some reason, they switched it up this year. I guess they make a change every so often. Earlier in the day, I watched an interview with Jon Voight. They showed a clip of him winning his Best Actor Oscar for COMING HOME. It was presented to him by Diana Ross and Ginger Rogers so again, I guess they switch it up every now and then.

Re the Best Song nominees, I believe they were performed on the pre-Oscar special. I came across Leslie Odom, Jr. performing his song there and I assume the later clip I saw of H.E.R. performing her winning song came from that special as well.