What you've described is not the accepted interpretation of events. Several people would benefit from your perspective, if you'd like to share the details. It is generally understood that you agreed that if Brom was fingered as the assassin, you would cop to it, IC. Is that not the case?
As to being accused of "cheating", I'm not even sure what that means in this context, but again: provide the details and I'll ensure that everyone involved is shamed. Apparently, that also means me.
Matt stated he had little time to play around this time, and that if anyone wanted to roleplay rebellious actions or attempts on the life of the King that they should not pull punches because he had low activity at the time. I contacted him privately, and said I'd be interested in such. After some short discussions, we agreed that I would work with him in developing an RP initial plot of events concerning an attempted assassination of the King.
Matt would have full control over the success or failure of my attempt on the King's life as well as to how much and of what type of evidence would be found against Brom for any attack. He'd also determine whether or not Brom would eventually be caught, or whether he'd get away with it depending upon how players pursued the events and sought out clues. I would supply the initial set up of the plans, and background of events as well as probably evidence which could either lead to or away from Brom. He would roleplay all events for both of us, so that no OOC interactions would dictate who may have been the culprit. If in the end, the evidence pointed towards Brom, I would verify all evidence in an OOC manner as legitimate IC acceptable evidence and then proceed to roleplay IC from there.
I'm going to skip over the fact that Matt's leaving caused a few problems in how this issue was perceived and many complications arose, when the switch in who was my contact changed. Namely, a lot of the interpretations were different, but I went ahead with things because two or three players had decided to invest time in the RP and I wanted to not let them down.
In the end, evidence pointed towards Brom, and so I verified in an OOC manner to all players invovled that they could act on any such evidence they found and that it was acceptable IC evidence.
Brom however, as an IC character never admitted to any action, and I see no reason as to why he should admit to such, when it is a guaranteed death for the character should he have done so. He was tried based upon the evidence found against him, and we went forward with the IC events from there. I never as a player guaranteed that I would admit IC to committing a crime, but would simply allow IC events and roleplay to carry out from there. All evidence was considered acceptable to be used against me, but I did not choose to have Brom commit suicide by stating he killed the King. Some people thought that I should have done so, and that I had originally agreed to do so, but that wasn't true. More confusion occurred during the transfer of my contact, and that didn't help things of course.