Chad admitted that he fucked up and overreacted. But he believes that it's MCs letter... still. He beat up MC because of HIS letter, not because of the fucking Dawe. But he, obviously, said to Dawe that it was because of him, because he can't show his letter and explain why he did that. Dawe thinks Chad did it because he wanted to stand up for him.
I don't think Chad still believes the MC was behind the letters. He initially acted on impulse, thinking it was the MC, but later when he calmed down, he realized he had the wrong person. In Burke's office, the MC has an opportunity to defend Chad... "Professor Burk, you don't have to do this to him. Can't you let it slide with a warning or something?". If you chose that, Chad gives the MC a look, as if to say... "are you serious?'. As far as Chad knew, the MC was the one who had gone to Admin about the incident. So, now the MC is not wanting him to get in trouble and removed from the Alphas?
Later, we see that when Chad shows the Arieth photos to the Alphas, and Anthony points out that it couldn't have been the MC, Chad says... "Yeah, I don't know. I wasn't thinking when I beat him up.". If up to that point he still wasn't sure of what to think about MC's involvement in the "blackmail", he's surely figured out by now that he wasn't.
Then, at the HOT party, in Mel & Sarah's room when Alex tries to start shit with the MC, Chad stops him & says... "Bro. Stop it. He's cool.". He wouldn't have said that if he still thought MC was behind the "blackmail". Of course, he wouldn't want the Alpha's to get into trouble, but you don't say "He's cool". You say something like... "Dude, you know we can't fight. So, turn around & get the fuck out, freshy!".
And further on in the game, the MC burries the hatchet with Troy. If, for some reason, Chad STILL was thinking the MC had been behind this, then he surely would have let his lover in on everything that had happened... and Troy woudn't have said his differences with MC are all in the past.
We don't really know what was in the "Chad" letter... as far as photos or what was written. If it was indeed blakcmail, and Chad was threatened to be exposed... what was the demand? Was there even a demand? ..."Do THIS, or else!" The "Dawe" letter was apparently just to instigate more friction between the two frats, but with the "Chad" letter, what's the point of "Is this the secret you're hiding?" ...without some sort of demand? As far as we know, there wasn't one. I suppose there could have been an attempt to pin it on the MC (even though most of the people don't even know who the fuck he is yet) ... "Break up with Sage, or else these photos will get out.". That seems kind of stupid & petty, especially if the MC is not on the Sage path. So, it makes you wonder if there was any sort of demand, or if the intent of the "Chad" letter was the same as Dawe's... just to heat up the rivalry. Whoever was threatening Chad, hasn't exposed him yet. He knows that if it was the MC & the DIKs, he would have been already. Pictures would get out, the Alphas would find out & Chad would be ostracized.
I just keep going back to Mel & Sarah. Them two are some shady bitches.