Multiple Images in ACR

I see what you want: keeping the ACR 'session' open while sending selections of the batch to editor and then coming back. Unfortunately, an ACR batch editing session is necessarily ended by the options 'Open' or 'Done'.

The alternative workflow is based on closing temporarily your ACR editing session by selecting all and clicking 'Done'. All individual edits, even partial are saved. To reopen them that means reopening the session with the same batch of files. No big problem with the organizer if you assign a common temporary keyword or otherwise if you work with subfolders. You can also assign star ratings to distinguish which files to reopen in the next session.

Note that if you are editing jpegs from the organizer, they won't open in the ACR the first time, but once you have quit ACR by 'Done' or 'Open', they will open in ACR just like raw files.