OK, I got it going here, as follows ...
The new "Allow Re-Edit" option has three settings:
Yes - the current behavior. The lock owner can re-edit at will - so open as many copies of the same record for edit as they want, and they can all be submitted. Which of course means potentially overwriting changes between instances of a form.
No - Can only have one form open for edit on a given record. It won't let them open a second one for edit, and it checks on submit to make sure the lock token in the form data === the one in the database for that row (so the form being submitted is the instance that has the lock), to prevent any spoofing or corner cases where they might somehow manage to get two forms open.
Exclusive - they can re-edit (open as many forms on the same record as they want), but only the last one opened can be submitted. Basically, when you open a form for editing and get a lock, we write a token (actually just the microtime and userid) into that row for the lockrow element data, and we also include that token in the hidden form data. So we know on submission if the submitted data belongs to the from that got the lock.
In all cases, submission is also subject to the timeout, so even if you have the lock, if the TTL for the lock has expired, the submit will fail. That's because once the lock is expired, someone else could have edited and submitted the record.
I've added a whole bunch of informative messages, which I'm open for suggestions as to how to word.
On form load, you'll get one of these, depending on what "reedit" mode, and whether you have a lock, etc.
Anywhere with %s shows the minutes left on the lock.
PLG_ELEMENT_LOCKROW_RELOCKED_MSG="You already had a lock on this record for editing. The record has been re-locked, but if you have it open in another browser tab, you will still be able to submit it from there. NOTE that this can cause data loss, if you submit multiple forms for the same record, as subsequent submits will overwrite data."
PLG_ELEMENT_LOCKROW_OWN_LOCKED_MSG="You already have a lock on this record, probably because you have previously edited it and haven't submitted it, so you cannot edit it until the lock is released. You must either submit the open copy of the form (if it exists), unlock this record in the list view (if your admin allows this), or you must wait till the lock times out"
PLG_ELEMENT_LOCKROW_LOCK_EXPIRED_MSG="This record had an expired lock, probably because someone edited it, didn't submit it, and the lock has expired. The record has now been re-locked for you. Any other forms open for this record can no longer be submitted."
PLG_ELEMENT_LOCKROW_LOCK_LOCKING_MSG="This record is being locked for your use. Nobody else can edit this record till you submit it. The record will stay locked for %s minutes. If you don't submit within that time, the record will become unlocked and editable others, and you will no longer be able to submit this form."
On submission, you'll get one of these (or nothing, if no problem):
PLG_ELEMENT_LOCKROW_SUBMIT_TIMEOUT_MSG="This record was locked for edit by you, but the lock has timed out. Your form has not been submitted."
PLG_ELEMENT_LOCKROW_SUBMIT_NOT_OWNER_MSG="This record was locked for edit by another user. Your form has not been submitted."
PLG_ELEMENT_LOCKROW_SUBMIT_WRONG_LOCK_MSG="This record was locked for edit by you, but in a different instance of this form, probably because you opened it for edit in another tab and didn't submit it. That lock has not yet expired. Your form has not been submitted. You can unlock the record by submitting the other form (if it exists), or unlocking it in the list view (if your admin allows this), or by waiting for the lock to expire in %s minutes."
PLG_ELEMENT_LOCKROW_SUBMIT_WRONG_LOCK_EXPIRED_MSG="This record was locked for edit by you, but in a different instance of this form, probably because you opened it for edit in another tab and didn't submit it. Your form has not been submitted. The lock expired %s minutes ago, so you can re-load the record for edit now, and will be able to submit it."
PLG_ELEMENT_LOCKROW_SUBMIT_ERROR_MSG="Record locking is enabled on this data, but your submitted form does not have the correct lock token, so your form has not been submitted. Please contact the administrators."