The outside pilot could get back inside if the inside pilot was incapacitated.
_________________________________
How?
check post #112
I'm not a pilot, but my guess would be simple: by using a key that he has, or entering a combination that he knows.
A generic door, anywhere, can be blocked from entering by various means. You can change the lock's code, for example. Or you can use a secondary lock (a deadbolt, or a bar) from the inside. But you have to mean it - this cannot be done accidentally.
Also people are saying that a flight attendant would have to be present if one of the pilots leaves. The audio recording would have that conversation. If there is nothing but silence, it might mean that there was nobody else in the cockpit. This violation of rules could have been seen as harmless on short routes, where the whole flight takes under two hours and is routine to the crew.
Also it is possible that the terrorist (if that's the case) could have been waiting for just such a day when the other pilot leaves and all flight attendants are busy. (Not impossible, as they just reached the cruise altitude, the safety belts are off, and everyone starts asking for tea, coffee, etc.) We can't know what those jihadists are thinking just before they blow up.
Can’t say, but there is a fail safe. This is suicide.