If I played an action because of his ability, can I play another action that turn?
[Source: WotC]
Yes.
If Autobot Mirage attacks in bot mode and I draw 3 whites so he's untapped, does my turn end with the attack, or do I attack again as no one would be tapped? Also, if I must attack again can I use someone else to attack?
[Source: WotC]
Mirage does not give you an extra attack. If your opponent has no untapped characters, you would continue attacking with all of your untapped characters in any order, including Mirage.
Let say I'm attacking with Mirage and flip 2 black pips and 3 white pips. As far as I know, my attack is 5
Pierce 2 and then his ability triggered and I untap. Then I attack again and this time I flip 1 white pip and 3 black pips. Is my attack 5 Pierce 3 or is it 5 Pierce 5?
[Source: WotC]
If I am playing Autobot Mirage in his Bot Mode and I hit three white pips while attacking, he untaps. Does this mean I get an additional attack this turn or is he just untapped for next turn? The only reason I ask is because his effect states "during his first attack" so I felt it implied he could attack more than once that turn.
[Source: WotC]
Ordinarily, he would only get one attack and then untap. However, if your opponent is completely tapped out, Mirage and any of your other untapped characters will get an additional attack.
If
Cornered! were played on an Autobot Mirage, allowing a player to play an action during their opponent's turn, and they played an
All-Out Attack; could they attack with all of their characters during their opponent's turn?
[Source: WotC]
No. A player cannot attack during an opponent’s turn.