Is it sufficient for someone who forgot the supplications for entering and leaving the lavatory and then remembered them while inside, to recite them in his heart, and will that have the same effect in warding off devils and jinn?

If a Muslim forgets the prescribed remembrance (dhikr) for entering the restroom, he should seek refuge in Allah in his heart, not verbally, according to the majority of scholars. Some scholars have permitted the remembrance of Allah absolutely inside a restroom designated for relieving oneself. As for places not specifically designated for relieving oneself, one should recite the remembrance when beginning preparations for it. Remembrance by the heart takes the place of remembrance by the tongue when the opportunity for verbal recitation has passed.