If you are staying in a hotel, the hotel would have registered the TM30 online. If you are in private accommodation then you may be in trouble. You should not leave the extension until the last day. Most offices let you do it anytime in the last 30 days. Jomtien will only allow the last 7 days.
If you use one of the many border-run travel agencies then they virtually guarantee you can come straight back in as they have 'special' arrangements with border staff
Onwardsticket.com the ticket is only valid for 48 hours but as the embassy are not members of the travel industry they don't have access to flight passenger names
you are correct a METV is valid for 6 months but if you leave and return just before it ends you can get nearly 9 months in Thailand. Please note the 6 month validation starts the day the visa is issued and not when you enter Thailand so don't buy to far in advance