The scripts in my game are very vunrelable and very fragile. If someone was to join my server with someone already in it since it's a 1 person limit player game if someone really exploited to join the game All the datastores would be messed up but is it possible?
No. Roblox's system makes a local copy of the game on certain games with a limit of 1 player / server in order to prevent lag and certain bugs including exploiters. If an exploiter tries to join your game, he can join the same server but he would be in a different game since Roblox made a copy of the game locally for everyone. You are safe because of Roblox's annoying updates.