-What are your predictions about what Mueller will find ("collusion", obstruction, etc.)?
Trump will be accused of Obstruction of Justice in a report sent to Congress. Mueller will find conspiracy with Trump campaign members and the Russians. Since the indictment of the Russians for the DNC, DCCC and Clinton campaign hack was so detailed, Trump campaign members who conspired here in America with the Russians don't stand a chance.
-What pieces of evidence are the most important?
-Cohen paying the hackers (if Mueller can verify that. It was mentioned in the dossier)
-The Trump campaign knowing the Russians were hacking the DNC. (As mentioned in the dossier)
-Roger Stone's contacts with Guccifer 2.0 and Wikileaks.
-Don Jr's contacts with Wikileaks and the Trump Tower meeting. Even though Don Jr, Manafort and Kushner was supposedly disappointed when the Russians didn't come through with the dirt on Clinton at the meeting, they probably made plans to get the dirt later on.
-Manafort managing the conspiracy through intermediaries.
-Does "collusion" have to be successful to be considered a crime?
Yes. The legal term is conspiracy.
-Will Trump himself be charged? If not, who will be?
Trump will not be charged because of DOJ policy that a sitting President cannot be indicted. However, the Trump Organization
can be indicted for Money Laundering with the Russians. The people charged: Manafort, Cohen (might try his hardest to cut a deal to escape the wrath), Don Jr, Roger Stone and possibly Kushner. Flynn may have immunity if he had any involvement due to his cooperation. I'm not sure what will happen to Carter Page & Jeff Sessions. Sessions might get off free somehow.
-How will they be sentenced?
Trump will pardon Don Jr, Manafort and Kushner. Flynn, Gates and George Papadopolous will not get pardoned because they turned against him. If Trump is removed from office, Pence will pardon him.
However, Manafort, Don Jr, Kushner and Trump Sr may get the hammer from New York State Law. There is currently a bill in the New York State Congress that would allow New York State to charge anyone with state crimes after they've been pardoned from federal crimes by Trump. As we know, only the governor can pardon state crimes.
Here is a link to the bill:
New York State Assembly | Bill Search and Legislative Information
The bill has been sitting there for now, but when Trump starts pardoning people I expect that bill to get passed quickly.
-How will Trump react to the findings?
Call it fake news. Start abusing his power, doing wild shyt and go down kicking and screaming. Wouldn't be surprised if he did something outrageous like demand that Obama or Clinton be arrested.
Might even tell his fanbase to come to Washington DC to riot.
-If referred for impeachment, how will Trump behave in front of the Senate during impeachment hearings?
Like a child. He'll embarrass himself in front of everyone and do the classic Clinton deflection and do the white/Trump victimization thing.