Mumbai, Delhi and Bangalore airports are the only places where you could get buy a sim card as a non Indian but it could be a bit of a hassle. If possible it could be worth while having a sim if you would like to travel as most of the apps like cabs etc would require a local number. The other option is to get an eSim before you land from a 3rd party provider. I usually use Airalo (https://www.airalo.com/) the only caveat is it only supports internet and you can't make calls to or receive from.

Suggestion for the attire for each of the events is mentioned in the home page. You are free to wear anything but try to avoid white and black color (except black suit is fine for guys)

We are trying to coordinate it with the event manager to find a solution. We would either try to arrange transport to take you to the right shop and/or provide you a list of shops for you to explore on your own. Do keep us in the loop if you are interested.

Major players like booking.com operate in India. There are some local websites like makemytrip (https://www.makemytrip.com/) which have very good packages and they could customise it for you to suit your need over phone. The same goes for hotel and internal flight bookings, it is a one stop soliution.

India has gone predominantly cashless with it's homegrown mobile payment system UPI. Foreign nationals can get UPI at the airport (https://www.npci.org.in/what-we-do/upi-one-world)(https://ebixcash.com/) but it won't be linked to your bank account and there is no option of topping it up online later. Most large places would accept cards, most places would accept cash (begrudgingly).

Tap water is not potable. Drink packaged water or from a filtration system.

It is not needed but there are some suggestions if you would like to be careful, https://vaccindirekt.se/en/pre-travel-vaccinations/asia/vaccination-before-the-trip-to-india/

It depends which part of India you are visiting. Delhi could drop to 2 degrees at night during December.

https://www.irctc.co.in/nget/ Overnight, vandebharat vistadome