If you go for drip, have a ventury also installed. you can feed water soulble fertiliser thru drip.
Foliar neem oil spray is good for sucking pest or stem borers or fruit flies etc. But for rootgrubs neem cake is advisable(takes a while to act) or Metarizium mixed with FYM(works fast). Neem cakes also good source of nitrogen.
Point 5 is critical issue, you have to keep an eye on other plants, if it is spreading then only it could be virus(points1-4 dont spread to other plants). you have to burn the affected plants away from your farm. Touchwood this may not be your problem.
On seeing the Image of your Plants, it seems that Boran Deficiency might be there wile attack of thrifts.
The environment must be weed free and there should not be water lagging and and should not be
crying for water particularly for the Papaya plantation.
PAPAYA, needs lot of sun, full day of sun if possible, 2 or 3 hours of sun wont do it= plants will grow ok but then it will die as yours in the picture, due to fongus, virus as root system dies because of escecive moisture lack of sun, and you need to know your soil, water once a week or every 2 weeks, do not water evey day as you do not know your soil,
plant 10 papayas and water them differenty, #1 every day, #2 every two days, #3 every tree days, #4 every four days, # 5 every 5 days, and so on, this way you will learrn your soil water retention. use two liitters of water per pllant…use some aphid repellent every week, or Dog Flea Shampoo= 2 spoons on a galon of water and sppray those suckers away.