A serious concern indeed! I greatly appreciate your taking the time to bring it to my attention, and can only apologize for my error in failing to consider the need ahead of time - it is no purpose of mine to cause anyone distress by my actions here, and I've added content advice prior to the beginning of the story.
As far as "kicking off what appears to be an ARG" goes - I hadn't considered it in precisely such terms, but I suppose that is more or less what I was doing, isn't it? Though I feel I've done rather a poor job of it, and would have done many things differently had I thought it through more ahead of time, rather than acting out of a sense of desperation complicated by the effects of a prolonged high fever. I think the "alternate reality" in this case must thus have existed entirely within the confines of my own skull! Next time, if there is a next time, I'll try to do a better job of making the game's alternate reality more accessible to people other than we who live inside this particular head, and also of making it possible for those who take an interest to know going in whether the subject matter is anything with which they choose to treat, rather than risk putting someone in a place where they do not choose or care to be.
Perhaps that latter necessity will, as you say, make it more difficult to maintain narrative verisimilitude - if so, so be it; I'm not sure I am exactly a "serious writer", but if I'm going to engage in fiction of this sort, then I'm going to do so in a way which does not risk harm to its audience. No doubt this will be a challenge! It is one which I will develop sufficient skill to overcome - and in the meantime, I'll from now on use proper care, to err on the side of excess caution rather than that of excess risk.
Thank you once again for your kindness in bringing this error to my attention! I would not be able otherwise to repair it, or to ensure I don't err the same way again in future.