1) We have adaptivity in Corona, which should help in cases exactly like this (some part of rendering is noise-free, some specific object/material is noisy). Additionally, adaptivity has been improved in the most recent dailies, so it may be interesting to check if you can feel the difference.
2) Bucket rendering is just another way of displaying what is being rendered, so I am not sure if they would help in this case at all. In the older versions where there were buckets, they would produce exactly the same output if the same render settings were used.
3) Could you post screenshots of the material setup and example renders here?
4) Could you send us the scene here
https://corona-renderer.com/upload and let me know what's the file name? Maybe something could be easily optimized in the scene or in Corona.