Can I use the second part of Misty & Lorelei to discard 5 cards if I have not already used my GX attack for the game?
No, you cannot. If you haven't used your GX attack yet, you can't discard 5 cards from your hand.
Source: Cosmic Eclipse FAQ (2019-10-18), TPCi Rules Team (2019-10-31)

Relevant cards

Note: All applicable cards may not be listed below.
SM12_199
SM12_199