Maybe this topic can give you some answers in general:
To answer some of your specific questions:
I would not use a SPA or “Various Artists” when exact data is available. Just take the writer of the script as AC for the release and use relationships to credit the cast / individual actors. A good example is the following audio drama:
Do you mean the form value “script” below “language” or the credit “Script: Hiro Akizuki (秋月 ひろ) (RUN&GUN), Hiroomi Takamiya (高宮 宏臣)” from your example? I’m no expert on Japanese script, but for the credit I would use the AC and the “writer” AR.