Liepard

Say I have Liepard active and Raikou-EX and Victini on my bench. I attack with Liepard's "Assist", flip heads and choose to copy Raikou-EX's "Thunder Fang" attack but flip tails for "Thunder Fang's" paralysis part. If I then use Victini's "Victory Star" Ability to re-flip the coins, assuming I get heads for "Assist" could I choose a different benched Pokemon's attack or would I have to choose "Thunder Fang" again?

Since Victory Star rewound all the coin flips it's as if the Assist attack never happened. So yes, you could choose a different benched Pokemon's attack, if you flip heads for Assist of course.

If I have a Liepard with 3 Darkness Energies on it and I use "Assist", can I use Charizard's "Scorching Fire" attack despite not having a Fire Energy to discard?

Yes, you can. The damage amount is done regardless of whether Liepard has any Fire Energy attached to it or not, but if you do have a Fire Energy attached to Liepard you must discard it.