Once you put it back into the Active Spot, you will be able to use Megaton Cannon for two consecutive turns as Retreating removes the attack restriction from Megaton Cannon
If your goal is casual play or playtesting, proxies can be a reasonable alternativeas long as they are clearly disclosed and used ethically
For the Synchro Summons, these are what the Deck commonly brings out
On top of all of that, the Charizard ex card being spoken about is one which has Terastallized into the Dark-type, thus bringing the novelty factor into the picture
Similarly, if all of the attacking player's other opponents leave the game before Suppressor Skyguard's last ability resolves, the condition will no longer be true when the ability tries to resolve, the ability won't resolve, and damage won't be prevented
You can only activate 1 "Underworld Circle" per Duel