Is it better to invest in US than to invest in India considering inflation rate and devaluation of INR

Inflation in India is of no consequence to you. As you are looking to keep the funds ultimately in US you should calculate the returns in USD. I.e. value of USD investment at investment exchange rate and gain in investment converted into USD at the prevailing rate.

Fund expenses are factored in returns. There is no separate expenses.

Edit: It is advisable to save in currency of retirement. It removes the currency exchange risk.

Answer to your 2nd question.

Few people notice the opportunity cost of emerging economy compare to any industrial country (USA, EU, Japan, China, ). Besides inflation, you should calculate the opportunity cost if you invest equal similar index ETF of any industrial country. For example, US S&P 500 index past 10 years gains is average ~ 10.27% per annum. While the rupees to dollars fells from 46:1 to 68:1 (47% different). After account for the exchange rates loses, you will a big income loses.

Industrial country

1. USA is still the world biggest research and development spending country.
2. The leading R&D position means the world still depends on a lot of core technologies
3. USA is still the world biggest consumption country
4. Bigger capital
5. There are stronger regulation and option to distribute the capital market risk
6. An equal percentage gain on US index is proportional to the counterpart currency exchange.

Emerging economy

1. Small capital company involved in export will show better growth
2. It is easier to manipulate on cheaper equity and properties.
3. A country like India has a potential huge internal consumption market
4. Financial and many industries are controlled by oligarch which allow them to rent seek on the internal economy.
5. Like many emerging economy, India National Pension fund that collect tons of money might subject to abuse to use to manipulate the market, e.g. something similar to Japan Price keeping Opearation in the past.

R&D is a crucial factor to ensure a country sustainable growth momentum. But it is possible if the country has policies to ensure the country to encourage companies to do it, produce enough educated people and enact a large employment market. Take South Korea as an example, although R&D spending is one of the world highest spender, the overall Chaebol structure limit its industry diversity and creativity. I.e. most of the South Korea core production component must import from Japan.

You can either diversify your investment to two countries stock or bet the USA market in the long run.