Hi @manghig, I'm replying to a variety of your posts and comments, partly trying to make sure other forum threads don't lead others to confusing outcomes later.
As you saw in my other response Deleting media from Central submissions - does it reduce database size - #7 by ktuite "clearing" a submission attachment and "deleting" a submission attachment are the same concept.
I think it should be possible in your case to reduce your database size by clearing submission attachments but not deleting the entire submission. You've identified that the deleting in Central usually just means "soft-deleting" and you also need to either wait for or manually trigger the purging process. The problem you're having with the purge script is that it is no longer called purge-forms.js, just purge.js. This is the kind of small change leading to big problems that I would personally find very frustrating, so I'm sorry if we've caused that for you! I just updated another thread ODK Central - How to delete a submission - #40 by ktuite to correct this there, too.
I'm now very invested if this works for you:
- clear submission attachments (see blobs still exist in database)
- run purge script
- see that there are now fewer blobs in database taking up less space
- vacuum or whatever needs to happen to reclaim that space
I also think the daily purge script should have deleted/removed any blobs for attachments you previously cleared, without the 30 day wait.
Edited to add: Do your submissions have multiple versions / have they been edited? We just realized that if that is the case, clearing the submission attachments will only work on the latest version of the submission, in which case the underlying blob wont be truly unlinked in the database and wont be cleaned up by the purge script. 
I've put this in an issue. Maybe we need a more powerful attachment delete.