Hi
1. Valid. But exemption will be under section 54 and not under 54F.
2. If property is registered in May 18, pay capital gains immediately of exceeding 10000 to avoid advance tax non payment interests, or at the time of return filing. Return filing will be upto 31st July 2019.
3. (154367/2)*2.72 will be your share in costs.
# I assume the inherited property was a house property and not a land.