It is possible for C9 and G2 to make playoffs, with these 3 (if and only if) conditions:
C9 beat G2 (any score, though G2 getting a map or not determines if G2 is 3rd and C9 4th or vice versa)
MIBR beat Envy (any score)
Lev beat Loud (any score)
I think only the 1st match is difficult/an upset so this is extremely doable imo.
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Why this works:
Envy will be 1-4 and thus eliminated immediately. This is so they can't cause the 4-way tie that eliminates C9 no matter what.
The resulting 3 way tiebreaker between the 2-3 teams: (Loud G2 and C9) first begins with H2H, which they all score 1-1.
Then the H2H map differential will give C9+1/0, G2 +1/0 (both depending on their match score), and Loud -1.
This because G2-0'd Loud and C9 lost 1-2 to Loud.
That's all, I hope C9 vs G2 is a fun matchup for and between them, and they both make it.