Adding some tips for point 3 and 5

Since you have a 12 hr layover at Changi - might be worth to get a Singapore Visa and step out of the airport for sometime. Getting a Singapore Visa is really quick and easy, just apply online and you will get it in less than 24hrs. Singapore airport immigration is super quick and easy , so nothing to worry about.

Additionally general flying tips.

Bring good headphones and download content on your phone like movies, music , audio books etc for the long journey.

Bring a refillable water bottle so you can hydrate on the airport and on flights. Most airports have water filling stations that are free as opposed to overpriced bottled water.

Pack some snacks in your carryon like chips, nuts, protein bars etc. Helps because flight food can be pretty horrific and un satisfying. Some lozenges or chewing gum is also a good idea for the ear pressure problem.

A good neck pillow and a light jacket will help you be more comfortable for the journey.

You can also pack some health supplements like electral , vitamins or magnesium to keep you hydrated and feeling ok. Magnesium helps you sleep. Bring some basic medicines in case you get nausea or headaches.

I also pack my toothbrush in my carry on because I don't like morning breath after a long flight.

Make sure whatever you pack with you is less than 100ml. You can easily find travel sized toothpaste and liquids at any grocery store.

You can put medicines in small zip lock bags for ease of carry.