What would happen if an aircraft were to touch down on EMAS?
will the plane's momentum carry it on to the main part of the runway unharmed?


An aircraft landing on the EMAS must not suffer control problems.



According to an EMASMAX document


  FAA Advisory Circular (AC) 150/5220-22A
  ...
  
  
  No adverse aircraft affects in the event of short landing
  ...
  


The referenced document doesn't state this but does say that EMAS should be set back from the runway end and 


  must not cause control  problems  for  aircraft  undershoots  which  touch
  down    in    the    EMAS    bed.        




According to RUNWAY SAFETY AREA PROJECT - IGOR I. SIKORSKY MEMORIAL AIRPORT, 
there is only a three inch reveal at the transition of EMAS bed to runway. So a landing gear transitioning this part this part would likely not be catastrophic. I imagine crushed EMAS material still has some thickness.