You are looking at the wrong line, I think. The journey time from London Liverpool Street to Southend Airport station is 53 minutes. Journey time from Stratford station to Southend Airport is 44 minutes.
My apologies, my times were all from St. Pancras. Liverpool Street is indeed in the required zone 1, Stratford is not.
Taxi Ramsgate to Manston is 3 miles and about 6 minutes by taxi, quicker than walking at say Heathrow or Gatwick (for me at least).
Manston Parkway station is already funded, right by Manston Airport.