I just played Nexus Blitz with an AFK from the start, and...
...literally nothing changed. At all. Every event occurred as if that AFK were playing. That generally doesn't matter until you get to Prize Fight where the mode literally depends on whether or not the players win, and guess what? The AFK is still there, still included, and still loses.
This game mode is more unoptimized than Smash Bros. Ultimate's online.
[slayer-jinx-unamused]