Is it permissible for a Muslim residing in Western countries to pray Jumu'ah (Friday prayer) without the employer's permission, knowing that the employer would refuse and being certain that the assigned work will not be negatively affected?

One must go to Friday prayer, and it is not permissible to abandon it, even if your employer prevents you, because there is no obedience to a creature in disobedience to the Creator. It is not necessary to seek the employer's permission as long as attending the prayer does not harm the work. However, if it does harm the work, then he must be informed.