Is there a way to get the latest copied parts available from clipboard in goJS, before calling paste?
No, the storage is intentionally hidden by the CommandHandler’s implementation of its copyToClipboard and pasteFromClipboard methods.
But you can implement your own, so that you can do what you want. See, for example, how the LocalStorageCommandHandler extension does it: GoJS/LocalStorageCommandHandler.ts at master · NorthwoodsSoftware/GoJS · GitHub