I followed the San Diego Safari parks herd for many years and they held quite a few females out on the exhibit area and bred many!That’s a good point. Though Indian rhinoceros aren’t an inherently social species, Basel Zoo had success in keeping (related) females together (historically holding a large breeding herd); and I believe Whipsnade held their cows together also.
With this in mind, it’s certainly a possibility Dubbo would be able to keep a daughter with Amala long term in the event the calf was female.