About Chad, blackmail, Mel and Sarah.
Blackmail or not? Chad receives something (we don't know if it's a message, or a photo, or something else) and he considers it blackmail. Putting yourself in his shoes, if you receive a simple message saying "I know about you and Troy", it seems that the author of the message wants something, otherwise the info would have leaked. So we think of blackmail. And we don't know more about it.
For Mel and Sarah. They are friends with Sage and with Chad. They know Chad's secret. This puts them in a shitty situation: betray Chad's secret by revealing it to Sage, or "betray" Sage by not telling her. They choose the middle ground, saying nothing but pushing Chad to leave Sage. Their reaction in EP8 is fear that Sage would know the truth, and since they're Chad's friends, Sage might ask them if they knew, and why they didn't say anything, and so on.
Knowing that in the envelope addressed to Chad, there were also photos of Arieth fucking DIKs, we can exclude Mel and Sarah from the "conspiracy". Why embarrass Arieth? No interest here.
After all, who did this and why? Sounds like someone trying to troll Chad and Dawe to me. The latter by showing that his girlfriend is a slut, and for Chad by freaking him out about his secret. For the who, I think of a DIK member who had access to the photos with Arieth (which are taken by DIK, in my opinion); and for Chad's secret, either this DIK knows about his relationship with Troy (unlikely) or it's some kind of misunderstanding. If Chad received a message saying "I know your secret", he thought it was about Troy when in fact it could be about something else (for example the use of doping substances).
