How to report stock split (2:1) in 2017 (shares doubled), for Incentive Stock options that were purchased in 2016. There was no Form 3921 for stock split

You simply halve all the "per share" figures on the Form 3921 and double the share figure.  It's "as if" the stock had split 2 for 1 prior to the grant.  There's no "reporting" until there's a sale.

Tom Young

when these shares are sold what should be the date accounted as purchase date for all post split shares - Will it be the original purchase date or separately based on when I received each one - original shares and free shares (split ones)

All shares post split are deemed acquired on the original date.  You were not issued "new" shares, the "old" shares just split.