Yes - the Indian LG G6 variant does have dual sim functionality. In fact, it does have hybrid sim slot - means you can either use 2 sim cards or 1 sim card & 1 MicroSD card.
Yes, LG G6 does support VoLTE.
Yes, Indian variant of LG G6 does have FM Radio. However, Korea, Canada and USA variant of LG G6 does not have FM radio.
Yes, the Indian variant of LG G6 has special DAC for audio. The Quad DAC helps to improve the output of the audio via any headphone. It is a unique stuff.
The LG G6 is still a decent phone if you can find it cheap.