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.