What is the ruling on the prayer of one who doubts his prostration in the last rak'ah, then is almost certain that he performed two prostrations? And what is meant by the distance between the chin and the neck that must be covered during prayer?

The fundamental principle is that if a worshipper doubts the number of prostrations or rak'ahs (units of prayer), they should base their count on the lower, certain number, perform what they are unsure of, and then perform the prostrations of forgetfulness (sujud al-sahw). This is based on the Prophet's (peace be upon him) saying: "If one of you doubts in his prayer and does not know how many he has prayed—three or four—then he should cast away the doubt and build upon what he is certain of, then prostrate twice before giving the salutations." Accordingly, your prayer is invalid, and you must make it up. Some scholars are of the opinion that a person who is prone to doubt should act upon what they predominantly believe to be true and then perform two prostrations of forgetfulness after the salutations. In this case, you would not be required to make up the prayer.

As for the second question, it appears that "between the chin and the neck" refers to the area below the chin. The chin is the meeting point of the two jawbones, and it is not permissible for a woman to expose it during prayer.