You are correct, even with the crypto currency income, you are below the filing threshold amount and were not required to file a 2017 tax return.
2017 tax year returns are open for IRS review until April 15, 2020. This date is the date the IRS has to audit 2017 returns; if an inquiry into a 2017 return hasn't been made by this date, the year is considered closed and no action can be taken against a 2017 return unless fraud is suspected.
It is also the date that an amended return would need to be filed to claim a refund by a taxpayer.
Your cost basis in 2017 was $2,400. Your cost basis remains $2,400 unless your transfer to Coinbase required a sale to transfer to Coinbase.
