As Matthew suggested already, I don't think that Knox implies this at all. It's as likely to be a matter of courtesy than the thought of actually locking somebody out.
Regarding your thoughts on the key situation:
1) A "convenience key" could not be left in the interior door lock, since this would prevent a key being inserted from outside - in other words, this sort of arrangement would prevent anyone from being able to open the door from the outside with a key.
2) If you stop to think about it, there's almost zero sense in having a "convenience key" hanging by the door. The only time this would make any sense would be if the person exiting the house planned to leave the door unlocked behind themselves (e.g. taking out the rubbish etc), and even then they probably wouldn't want to risk someone else locking the door while they were outside (assuming they replaced the "convenience key" once they used it to unlock the door). If someone were going out properly, then the door would need to be locked once they got outside - necessitating a key. So they would either take this "convenience key" and hold on to it while they were out - thus ruining the whole concept of a "convenience key - or they would take their own key (and therefore use it to both unlock and re-lock the door as they left).
3) Most landlords aren't fond of supplying more front door keys than there are tenants - for obvious reasons. If each tenant is personally responsible for her own key, then there's accountability. If there's a spare "convenience key" which just floats around, this is clearly more liable to go missing - with security consequences. It's therefore almost certain that the landlord would only want there to be four front door keys in existence, and that each of the tenants bore personal responsibility for her own key.
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Some different thoughts on the key situation.
Apparently we have no way of knowing whether or not there were spare keys, or more specifically a spare key kept by the door in this instance, but in response to your musings on the likelihood of such a circumstance I have to wonder if things are so different in the U.K. or Italy in that regard than they are here, because what you suggest is certainly not the situation in the U.S.
Starting with your item 3).
Acquiring a copy of a key is no big deal here. It will set you back a couple of dollars, and can be done at any hardware store, any big box DIY store, and more than a few WalMarts or Targets. Not to mention lock shops. I can think of half a dozen places I can get a key made in a matter of minutes within a two mile radius of where I am sitting.
Speaking from my own experience, which encompasses a number of rental properties in a number of states over more than a few decades, generally a landlord will give a tenant one, or possibly two keys and if the tenant wants more they go out and get copies. The landlord usually doesn't give a rat's ass how many copies are made. They do make the return of
all copies a condition of the lease, but that's about it. Particularly responsible or accommodating landlords will simply switch or re-key a lockset between tenants, although I don't know how universal this practice may be. It isn't expensive, especially compared to the headaches of tracking, replacing or issuing new keys. A cheap entry lockset costs about $30. A decent handyman can swap one out in less than fifteen minutes. With cigarette breaks. Many of the better ones are designed specifically so that the lock cylinder barrel can be removed and replaced with nothing more than a special change key designed for that purpose. It takes about a minute to do. When I was turning over finished buildings to an owner I would often have a handful of them in a drawer in my desk in the office trailer. Often times even if the landlord doesn't choose to change the lock the tenant may do so all on their own, and simply give a copy of the new key to the landlord. This is not the least bit uncommon, either.
Most landlords or their service people have a larger stock of old replacement door hardware than they really want, much less need. It's the kind of thing which tends to accumulate when you manage properties.
Regarding item 2).
Double deadbolt locks are a subject of much debate in the lock industry where residential entry doors are concerned. Their primary ostensible purpose is to prevent unauthorized entry by an intruder simply smashing a door lite or adjacent window and reaching in to turn a thumbturn. An alternative justification is that thieves will be discouraged from fleeing the the door with something which they couldn't remove through whatever other means of entry they used.
The problem with the second thought is that most things which are stolen can be gotten out the way the thief got in. Things that are too heavy or bulky are generally not stolen by a single thief, and the same logic applies. If they got in through a window, and they're stealing TVs or other bulky items they can generally find a window to hand big stuff out to an accomplice. Once in, the means of egress increase dramatically. The front door is not the only hole in the walls.
The security advantages are seriously offset by the safety disadvantages. If someone is woken by a fire and tries to flee, the obstacle presented by a locked double deadbolt can and has been the difference between life and death. Fire safety professionals hate double deadbolts on residential entry doors, and universally advise against them. When their advice to avoid them is to be disregarded the fallback advice is to insure that a spare key is kept close to the door in case of emergencies. (Or to use a system like a "captured key thumbturn", which I won't go into now.) Most of the people I know with double deadbolts do this.
In addition many residents don't want to be bothered by finding or fetching the keyring they left in the bedroom or their purse or their coat pocket every time they lock up for the night, and keep a spare key convenient near doors with these sorts of locksets. Remember, the cost of an extra key is less than most people spend on a morning coffee at Starbucks these days.
All of these conspire to make it much more likely than not that an extra key will be somewhere in close proximity to a double deadbolt lock on a residential entry door. If anything I'd be inclined to think that it would be even
more likely in the case of the girls' apartment, since the disabled spring bolt and resulting need to lock the deadbolt merely to keep the door closed would mean that a key would be required simply to answer the door whenever there was any sort of caller.
Item 1). It is just wrong to say,
"A "convenience key" could not be left in the interior door lock, since this would prevent a key being inserted from outside - in other words, this sort of arrangement would prevent anyone from being able to open the door from the outside with a key."
This is entirely dependent on the type of lock assembly itself, as well as the type of cylinder being used in that assembly. These are often (usually) interchangeable within a manufacturer's group of locksets. If you believe that all double deadbolt lock assemblies will not permit keys to function in both sides of the door simultaneously then you are simply mistaken. Without knowing the specific details of the specific lock assembly on that particular door it is quite impossible to make the statement you did with any foundation in authority.
I'm going to come to your defense in the "probability" aspect of this, though. Although in the U.S. it is quite common, probably even normal for double deadbolts to accept keys in both sides of the lock at once, my reading into European lock hardware during the last time the subject came around led to the discovery that a common "Euro" style of lock cylinder does not. I am not going to go through all of that reading again, but there is every likelihood that the lock assembly on that apartment door was using one such.
Note that it did not
have to be, and we still have no way of knowing whether it did or not, but I would be unsurprised if it did. Not that it is at all germane to any other part of this question.
