Hi everyone hoping I can get some expert help on a validation rule I've been struggling with.

We have two stages in Salesforce ('Financial Review - LOI' and 'Financial Review - Lease') that only users with the Financial Review Committee permission set can move the opportunity out of (forwards or backwards). 
 
There are times when an opportunity is moved into these stages by mistake, in which case I would like all users to be able to move the opportunity back (to the preceding stage, but not the stage after). How should the validation rule be written?

Thank you so much in advance.
 
Here is the current syntax:
 
( ( ISPICKVAL ( PRIORVALUE ( StageName ), 'Financial Review - LOI' ) && !ISPICKVAL ( StageName, 'Financial Review - LOI' ) ) ||( ISPICKVAL ( PRIORVALUE ( StageName ), 'Financial Review - Lease' ) && !ISPICKVAL ( StageName, 'Financial Review - Lease' ) ) )&& !$Permission.Financial_Review_Committee