The initial prerecorded message (from Percival) does explain the purpose of cameras (Tortoises are unable to move if something’s looking at them, due to social anxiety) and closing the door (making the other tortoises + bri’ish person get bored and go away before they can wake up Kevin’s pet tortoise), and the ‘only able to look at the cameras when facing the door’ is intentionally ‘clunk’, because ‘facing the door’ is intended to be the default position for the player.
The only reason to turn around (and look away from the door) is to get Percival to shut up when his terrible precorded singing starts (so you don’t wake up Kevin’s pet tortoise), and, due to the regularity of that happening, the player is forced to turn around regularly. If it wasn’t for that, there would be nothing stopping the player from just leaving the door closed constantly and cheesing their way throughout the Unspecified Period Of Time. Giving the player access to the cameras when looking away from the door would go against the intent of making the player feel exposed+vulnerable when shutting up Percival’s terrible singing (as, by having access to their first line of defence (cameras) when looking away, the player wouldn’t have any real reason to look back at the door as quickly as possible, potentially removing the intended sense of panic the player should have when trying to work out where the ‘shut up’ button is and trying to click it.)
I did try to make things a bit more manageable for the player by making everything that does appear outside of the office make distinct noises (different to their ordinary movement noises) so a player can react to them (and close the door on them) even if they’re not currently looking at the door, but I suppose it is the sort of thing that a player might slowly work out from a long enough period of trial and error. Anywho, there is also this guide which attempts to explain how the enemies work in a bit more detail as well.
(And yes, Tortelvis’s appearance changing whenever you look at him is a thing in this game. It’s intended as a mildly-amusing thing that makes a player feel mildly concerned, but it doesn’t have any real gameplay implications)