As I understand it the concept of not being able to /hide in the same square twice was originally a balance mechanic rather than an immersion based change. Problem is this is balance by frustration essentially. This seems to be a strange fix since as we all know you will immediately move out and back into the square to repeat backstabs.
This mechanic is mildly annoying for PvP but really annoying for leveling and PvE. It seems like most players are in the same boat of dreading leveling Assassin's due to this mechanic. It drags mobs around every single turn, delays leveling, and is generally annoying as hell.
Builders are having to build cheeky fake room moving mechanics that instantly let you move out and back into the room without really leaving which is a clever work around but only applies to the room its built in. It is a great band-aid fix but why not just fix the core issue?
Make backstab a 2 endurance command. It is a 2 endurance command that either attempts to crit for 4x or 3x depending on the class. (You could call it something else for the bow crit I guess too or even rename the mechanic to something to indicate a more general critical strike?)
This could potentially leave room to tinker with the /hide and /autosneak mechanics if anyone wanted to do so. It is weird that sneak and hide are both stealth mechanics that behave a bit differently but with /hide being unnecessary for backstab you could combine the functions into something sleeker like /sneak essentially replacing /hide at one endurance cost for sneaking in the room causing you to become hidden. /sneak could have directional triggers for sneaking costing endurance.
The whole hidden vs invisible mechanic seems very weird that you can be invisible via sneak or fade yet attacked. Thematically and with gameplay reasons it would make more sense to combine them into just one hidden feature and balance as needed. Sneak could actually be a hidden state that you have to search for to attack but costing an endurance per tile. Fade can be balanced easily as well (afaik the attacking without breaking fade thing was fixed).
Devils Advocate Arguments:
This would buff classes that use the backstab mechanic since they would now be entering one command instead of four and bumps their dps.
Simple: Nerf the endurance regen a touch for Assy/Slayers to mimick the changes. You could leave Thieves out of it and let them enjoy a small buff and I'd wager the community would be happy. You could ship this change with a nerfed endurance regen to the Assy/Slayer class or simply wait to see if the buff is impactful enough to deserve a small balance nerf.
What about locked down rooms that are build to prevent more than one backstab? This would break that mechanic. Staff/Builders might want certain Event, OAD, or Quest rooms to not allow backstab for balance reasons possibly.
There are so many ways to balance this that are all pretty simple. Events? Don't allow the class. If you prevent Slayers from backstabbing in an event they won't win or enter it anyway - just ban the class. Or use a room that doesn't allow for backstab commands. For OAD, quest, or event rooms you can simply balance it by making a room not allow backstab or making the Mob take reduced damage from backstab. This all can be determined case by case as is needed. I don't think backstab is that crazy op but if it is deemed so just balance it like any other mechanic.