I am going to hold the celebrations until we are sure there isn't anything else on the horizon. He isn't getting a pardon and being declared free and clear of charges in the US is very difficult. Who knows what state prosecutor might want to bring new state charges. He may also be wanted as a material witness. If I were him I wouldn't set foot outside Australia ever again.
You can definitely get pardoned before getting convicted. Trump pardoned Stephen Bannon after he got indicted wire fraud and money laundering, so he never went to trial for that.
The pardon power seems to be very broad, and is most likely constrained by politics more than by statute. Set aside the idle speculation about a president pardoning themselves, pardons have been granted to whole classes of people for crimes you have yet to be charged with. Consider Jimmy Carter pardoning draft dodgers or Abraham Lincoln pardoning soldiers who fought for the Confederate army.
He is getting a conviction. This is a plea deal. He will be admitting to the crime of conspiracy after which he will be a convicted by the court. He will then be a convicted felon and could be pardoned, but I doubt that is really an option. (Not 100% on the felony thing, I haven't seen how this is being charged.)
