Our mysteries will surely need plenty of planning. And I can see Moriarty leaving small hints toward a bigger picture, mostly because he's got an ego and I do think, feels Holmes is a near equal, hence an amusing threat. In terms of the first case, something petty like something lost, or a missing pet, or some kind of threat? I'm thinking the obvious solution would clearly be wrong, and that there would be a few layers to the issue before it's really resolved. If Moriarty and Irene have somehow set up the mystery, it would have a few layers and maybe be used to test Holmes' skills of deduction?

Irene can vouch for Holmes as being brilliant and Moriarty knows of his reputation but I think Moriarty would be curious to still test him through Mary?

I do think Irene would step away from Holmes, Mary and Watson after the first case and simply check base with Holmes or Mary from time to time, after all the ladies need to sit down and have tea every now and then to make sure Holmes is kept at bay. I think we're on the same page in that regard as well. If Irene is too involved, it'll be weird since she's surely not a reformed criminal herself.