Can a batsman use back side of his bat to play a shot in cricket. Is that a valid cricket shot ?
It's perfectly legal but due to the uneven surface shots will not result as intended. The laws of cricket do not have any specific rule pertaining to this particular aspect of the game.
Please check this link
Yes there is no restriction in using any side of bat, even edges. There are though restriction on size, weight and shape of Bat. Check here https://www.lords.org/mcc/laws-of-cricket/laws/law-5-the-bat/