Yeah true, Purab wasn't in the courtroom,in long shots he couldn't be spotted but if situation is to be explained then purab too didn't have any PROOF at that moment; The contract was signed by JESSICA BEDI,tickets from mumbai to Goa;paris or back mumbai were booked by the name of JESSICA BEDI;so he too would have been called a feigned witness in absence of evidences like her parents.His witness can be used later with the back up of evidences.
As far as comparing with armaan is concerned ,that's the difference;Armaan is impulsive,Purab thinks before he speaks !!