I cant see hopw they can accuse you of cheating, they cna say you are in breach of the rules.
What they probably realised is that they implemented the rules incorrectly in the first place, thus they were partly to blame. They were also to blame for not reporting it. Thye would get into toruble for that, so by letting you off they saved themselves as well.
You have two options.
1. You now know you shouldnt be logged into your own account. Get to the exam early, remind the inviglator you are allowed to use the computer and allow them to set it up. That way responsibiliyu is theirs and they should log onto the correct account.
2. You can contact the exam officer for formal instructions and make them aware of the mix up, but if they havent told the exam officer, then it could raise more fuss.
Theres no point complaining unless, they decided to take action about it, which they dont seem to be.
Focus on your exams, complain after if need be, but for now, focus and dont worry. Let them set it up for you. You dont need instrucctions for that.