There's a certain portcullis in this mission - during an activation are monsters on one side able to open it to move towards the heroes?

Are monsters in RTL able to open doors if its not on their AI list of actions? or is it assumed its part of 'engage'

They are not able to open doors.

Monsters cannot interact unless it's listed in their actions...

and remember that a portcullis is an invitation to perform ranged attacks: You can count squares through them and have line of sight through them, but you cannot move through them.