I was circumcised as a teenager so that I would look like others in the locker room. I know first hand the difference between being intact and circumcissed.
I'll answer in order:
1) The frenulum can be left intact during a circumcision. Even so the foreskin holds about 20,000 nerve endings which you lose in a circumcision. Exposing the glans (head) also means it will be rubbing against clothes as you walk, run or play with your children. This is still irritating 19 years after the operation, though it was nearly debilitating in the two years following. Sex is definitely better if you are intact.
2) The glans doesn't become sensitive. It is normally sensitive. If you wish to make it less so then you can pull back the foreskin and walk around. This will be irritating and give a hint of what being circumcised would be like. Try using a silicon based lubricant during intercourse. If that doesn't work try a desensitizing spray/lotion. They're abundant in the adult stores here in the United States...I'm not sure about India.
3) A dorsal slit has few side effects, and will cure your issue instantly. Human skin however is very flexible and will stretch under certain conditions. The skin will stretch if you use a lube (and a desensitizing spray if necessary) and masterbate by pulling the skin back and forth over the head.