I recently traveled to Nepal via Turkish Airlines. So, here are my answers

1. Your question is not clear. They have four flights to Nepal in a week. But just one flight on each scheduled day. So what do you mean by "changing the schedule to the late one so that its on time"?

2. We requested to issue temporary visa so that we can go and stay at a hotel on our own expense. But, they denied on the ground that their govt. policy doesn't allow nepalese to get temp. visa at airport. They do have a hotel inside their airport but they always say that it's full. But, if you want to avoid this situation for your parents, you can get visitor visa to Turkey from Turkish Embassy at KTM beforehand.

3. You have option of choosing your meal while booking the tickets. We chose non-veg hindu meal. So they gave us chicken. We didn't have problem with the meal. You can choose veg hindu meal. You have to re-confirm about the meal one day prior to the scheduled flight date.

4. Worst Customer Service ever experienced in an international airport.

5. Don't have enough time to answer your 5th question.

Hope my answers were helpful for you to decide.