Sprocket change update !
Collected the bike on 1st October (Saturday), didn't have time to update the thread because of my hectic schedule.
1. The new rear 42T sprocket is installed. Have not been able to ride much after that (reasons mentioned in point no.2 and 3) but the change in gearing is pretty evident. My college(and the KTM service centre) is right on the Delhi-Jaipur expressway so thought of testing the behavioral change of the engine character but couldn't manage high speeds due to the extremely busy roads. First impressions are positive though, will get to know it better when I go back to Dehradun next week. Will attach the costs incurred.
2. I always test ride the bike for a short distance before making the payment. Went on a 1.5kms spin on the service lane itself. Instantly noticed weird vibration(not like the pulsating sensation you get when you brake hard and the ABS kicks in) in the rear brake lever. Felt the vibration everytime I applied the rear brake even slightly. Turned the rear ABS off just to confirm, same thing happened. I came back and brought it to the service head's attention. He rode it, along with 2 mechanics, and they tried to convince me that it is just the ABS kicking in. They checked the brake lever free play, rear pads and rotor and ensured that everything is fine. I didn't pay much heed to the issue and thought of trusting the service head and the mechanics. I thanked them for the job and headed back home. Lo and behold, about 2kms from home, the ABS FAILURE warning popped up (see attached photo). Immediately pulled over and turned the bike off, took a 2mins break. Turned it on and voila, no ABS FAILURE warning. I was happy and relieved, only to be disappointed a minute later when the warning popped up again. Got home, took the key out and was walking away towards the stairs when I noticed that the DRL was still on. Yes, the DRL was on even after removing the key. Put the key in and took it out again, it turned off, all good. I was extremely tired so slept it off. Had no other option other than to get it checked after I get done with classes and exams, i.e. 12th October. The bike was completely rideable without ABS so didn't stress much about it.
3. Rode the bike on Saturday and Sunday evening with the ABS FAILURE warning and no DRL issues this time.
4. Reached college on Monday 8:45AM, parked the bike and went inside. I have some weird habit (hope fellow BHPians can relate) that I take a look at my bike/cars from all sides whenever they're parked. Came out for a tea break at 11AM and 3PM and the bike was all good, DRL off. Class ended at 6PM, put the key in and saw that the instrument cluster was glowing very dim. Battery voltage was 5.5V

and the engine/coolant temperature bar was flashing red constantly. There was no scope of the bike turning on in this condition. Took the key out to see the DRL still on, I got frustrated. Checked the user manual, played with the fuses but couldn't manage to turn the DRL off. Called KTM Gurgaon ASC, explained the situation and they gave a tow truck's number. He didn't pick up. Pushed the bike half a kilometre to a roadside mechanic who could charge the battery. He refused to touch the battery terminals with the DRL on. Yes, the DRL was on after the key being out of the ignition for half an hour. No other battery shops were open nearby. Was losing my patience so started calling the tow truck's number and he picked up after 10-12 attempts. He refused to come to my location. I requested for the contact of any other person. He gave me a number and I called him up. I sent my location on WhatsApp and he promised to reach in 45mins. Onlookers were having the time of their lives seeing an exhausted skinny guy sitting on a Duke 390 with the DRL on wearing a riding jacket in this weather. The tow truck luckily arrived in exactly 45mins. Loaded the bike, headed home with him in his vehicle. Thanked him for the excellent service. He charged ₹1200 for 12kms, I had no energy to negotiate. Fellow road users were staring at the bike and pointing to the DRL(still on). Got home, parked the bike, rushed upstairs, got my allen key set and removed the headlight from the bike, DRL finally off after almost 2˝hrs, voltage shown on the instrument panel - 4.2V.

Yes, the headlight does look like a predator.
I have absolutely no time till 12th October. Will try getting the battery charged on my own next week, fix the headlight back on the bike and head to the ASC right after.
Any idea what could've triggered the issues I mentioned ?
It is sad to see such a beautiful machine just lying idle :/
PS - The battery is just 4 months old and there are no blown ABS fuses, I already checked.