Assume the following horribly contrived scenario:

• 8-player Free-for-All multiplayer game.
  
• AP casts Warp World.
  
• Everyone counts their permanents and shuffles up (as per steps 1 and 2 of Warp World).
  
• AP concedes (because reasons).
  

How does the game proceed?

Similarly:

• 8-player Free-for-All multiplayer game.
  
• AP attacks with Colossus of Akros (20/20 trample).
  
• NAP blocks with Fugitive Wizard, Glory Seeker and Centaur Courser.
  
• AP assigns lethal damage to the first two blockers (1 and 2 respectively), then concedes before assigning the remaining 17 damage.
  

What happens?

Are these types of scenarios (theoretically) possible at Competitive REL?

In your Warp World scenario, the remaining players reveal cards and put them onto the battlefield exactly as they would have if the active player hadn't left the game, because if a player leaves the game in the middle of resolving their own spell, the spell continue to resolve. In your second scenario, the current damage assignment isn't legal, so players back up to the start of damage assignment and proceed from there without the missing player and his or her creatures.

In both scenarios, whether the remaining players wish to never again play with the conceding player (because that player is clearly a rules-lawyering jerk for insisting that no, they're conceding during the process rather than before or after for no reason other than to make things more difficult and confusing for everyone else) is up to them.

For future reference, please keep in mind that posts to this forum get emailed to to a lot of judges, worldwide. Corner-cases like these that can and will never arise in actual games in actual tournaments where a judge would need to issue an actual ruling are not something that needs to be emailed to the entire judge community. If you would like to discuss corner case scenarios with other knowledgeable people, there are other venues for doing that, such as Rules Theory and Templating.