Can the scenario you are describing happen? Yes! Nowadays, companies are quite money conscious and if they can cut corners and save some money without affecting the work too much, then they will do it. Having said this, you must think about your options.
Let’s evaluate your situation; you are taking a leave for Eid vacations combined with your potential marriage. If it is feasible, then you can try to cut short your vacation and go for only half the amount. This way, it will be hard for the company to assess the quality of work this replacement person is doing and before they get comfortable with her in your absence, you will be back.
However, this is not a long-term solution. You can perhaps avoid losing the job to a less-paying person this time but the threat will always be there. The best way to guard against this is to acquire skills that will allow you to; a) become irreplaceable even with someone demanding a lower salary, or, b) be able to find another job easily with your professional skill-sets.