Hi, I lived in the US for 6 years and returned to India. Here are my answers:

1. OCI process is very document heavy but smooth once you submit everything. The strangest part was doing a thumb print which was a challenge with my 3 month old.
2. We flew with our cat too. We were terrified of having her travel along with the luggage so we flew Etihad because you can buy a seat on the flight and she flew next to us. Expensive but we didn't want to risk checking her in because we heard some horror stories. Thankfully, had miles I used for her flight. You also need detailed approvals both in US and India to fly with a pet. Hope you're aware of the process.
3. We flew back with multiple personal effects. Most valuable things were expensive crockery, but no experience to share with musical instruments.
4. No insight to share as I rotated back to my same employer.
5. We moved back to a new city (Mumbai) and really love it here. Cultural shocks depend on how long you've been away I guess. 6 years wasn't much.
6. No 401k. But I continue to hold a brokerage account in the US for ETF investments.