How to roast peanuts on the gas stove without getting the brown spots?
Roasting peanuts in a pan on a stove cooks them primarily via conduction (i.e., the surface of the peanuts touching the hot surface of the pan).  Since peanuts are round, each part of the peanut must touch the pan for an equal amount of time during the cooking process to be evenly roasted.  That's nearly impossible without something like a barrel roaster (which very few people will have in a home kitchen).

Cooking with convection (i.e., hot air) will produce much more even results since the heat will transfer to all sides of the peanut relatively evenly.  Roast the peanuts at 175°C/350°F for 30 to 35 minutes, rotating the pan half-way.  You can also try putting the peanuts on a wire rack (as opposed to directly on the sheet tray), which will help the hot air to more evenly cook their bottoms.

If it does not interfere with your ultimate use of the peanuts, you can also try coating them in peanut oil before cooking (roughly 30ml of oil per kilo of peanuts).  The oil will help the heat transfer more evenly to the peanuts.

Finally, you can also use the Chinese method of deep-frying the peanuts in your kadhai; roughly 15 to 20 minutes.  Immersion in hot oil will cook the peanuts evenly.