Data Sheet Interpretation - "Insulation Area" and mystery Symbol
Am I right in my assumption?


The distance between the main contacts and outline of the contacts within the INSULATED is 2.7mm. The maximum pad length on a microSD is 1.6mm.

By specifying an area that must have resist, the chances of shorting is minimised 




  Is there something I should do to the layout in that area or will the
  solder resist suffice?


Resist should be fine but check tolerance stack up.


  What does the Symbol in the green box mean?


That symbol is a mechanical drafting symbol for positional tolerance 


  3.4 LOCATION
   - Position tolerances are the most common location tolerance. Concentricity and symmetry are also types of location tolerances. Profile tolerances can be used to control location, but these are covered separately.
  
  3.4.1 Position -The position tolerance symbol is a circle with a horizontal and vertical line drawn through the circle. See Figure 26.
  
  [source: Drafting manual. Dimensioning and Tolerancing Symbols.]