How should one reply to the Azaan if one forgot to reply from the beginning?

Q: If a person did not repeat the words of the Azaan immediately after the muazzin but only remembered at the end of the azaan at the time when the muazzin reached Hayya Alal Falaah, can he repeat the words from the beginning of the azaan or he should only repeat the words of the muazzin from hayya alal falaah?

Exchanging gifts

Q: There is a custom in some communities, that is; when we give some prepared food to our neighbor or somebody they always give back something which they prepared, in the same plate in return. The problem is, when nothing is found in their home they refuse to return our plate empty and say that, "I will give later"because of custom. Is this an act of sunnah to give back something as hadya in the same plate when one receives hadya from a neighbor or someone else?