الإجابة
Alhamdu Lillah
1- Replying to the Salam of others is better and more compliant with the Sunnah of the Prophet SAAW as spreading Salam is Sunnah as you know, and there is no special consideration or exception for those who recite Quran. However, replying could be by hand only without interrupting your recitation.
2- The work, Khatmah, means finishing the recitation of the whole Quran. Repeating one Juz’a of the Quran 30 times is not counted as Khatmah.
Khatmah has a special virtue according to some resources, like the saying of Abdullah Bin Masa’ad RAA : Whomever finishes a Khatmah will havean accepted Dua’a. And there is another narration that Anas Bin Malik RAA used to gather his family and kids when finishing the Quran and make a congregational Dua’a.