I'd support kurnal's points. Access shouldn't be a problem though, because the chain can only be on when people are in, in which case they are available to take the chain off to allow people access to risers, etc.
To answer Q2 the landlord must insist that the route remains available for use, it's within the area for which he/she is responsible. There used to be a story going around about someone who died in a fire because they couldn't get the chain off their front door quickly enough - was there ever any truth in that - anyone know? Also, this is in the landlord's area and he/she would have every right to remove an unauthorised bit of door hardware and claim compensation for the damage caused.
Q3 - Don't know what the fire service would say. It would depend what mood they were in. I can't see it's any different from the occupier having one on their actual front door. And that leads to a final point, why do they want to put this on this door and not on their front door? I can only think that kurnal has it spot on and the tenants are trying to claim a bit more territory as their own. With territory comes occupation: pot plants (no problem), pictures (not much problem), coat stand (not so good), bags of waste waiting to go out to the bin (bad).
In honesty, we can't judge, not having seen the building but, if you're the fire risk assessor, be wary of such a proposal and try and uncover the true intentions of the tenant - they'll deny everything of course, I would.
Stu