Corporate actions like MMEX's reverse split fall under state law - in this case, Nevada.
Nevada is a non-recourse state, which neuters shareholder actions against the board and management (Hanks and Lemons).
FINRA does not approve corporate actions - they just process them - FINRA can only delay the processing until the proper notice (10-business days from the finalized action) is exhausted and all of the forms and fee payments are met. Hanks errors in the Nov. 2018 split resulted in nearly 60-days of delay, turning MMEX into a zombie issue. Having said all that, FINRA can't block the action.
The sole entity with authority to step in is the SEC - the SEC could suspend MMEX, and/or de-list the issue from OTCM. That would require an investigation, and adjunct/administrative injunction.
The only other way to avoid the RS is for MMEX's board to reverse the action to change the corporate charter - revoke the amendment. Just like Hanks and Lemons took unilateral action, with no shareholder recourse to effect this RS, they could revoke it unilaterally - however, they won't - the reverse split is absolutely necessary to allow Hanks to continue his share-holder terrorism campaign of toxic borrowing.
The MMEX slow-motion train wreck is inevitable. Anyone holding at this point is just a sad, very sad (5x) bag-holder.