OK, so I had some time to analyze the hand history provided by OP.
During the hands when gd1990, caroline93, and MichaelDE are playing by themselves, there ZERO showdowns. So not 1 single showdown in the first 43 hands. That's a bit odd.
The first showdown happens in hand 65, or #242704944, after valentinos77 and AshPK have joined in. gd1990 wins the pot.
The second showdown happens in hand 70, or #242712360. MichaelDE wins the pot.
The third happens in hand 79, or #242712484. gd1990 wins the pot (the one in this
screenshot).
So you can see that showdowns only started happening after other players had joined the table. The longest streak of no-showdown, across all 867 hands, is by far the first 65 hands; 43 of which were only played with the 3 players suspected of collusion.
While other players were playing:
Showdowns between gd1990 and caroline93: 5
Showdowns between gd1990 and Michael DE: 11
Showdowns between MichaelDE and caroline93: 1
Across 700+ hands played together, there was only 1 showdown between these two players... That's pretty anomalous, don't you think?
That two players only make it to showdown 1 time in 700 hands is not so rare, nothing can be concluded from that. I find the above analysis more indicative. All in all it can indicate collusion. It could be due to variance, but quite extreme, that the players involved do not reach SD until more people join. I will review this case later.
