Thanks for putting so much detail in! The only thing that's missing is the setup for the high limit detector you have on DP_i1.
Your output point looks like a virtual point, so there is no excuse for it not accepting a set (unlike a Modbus point for instance, which could have the set request timeout). It is very weird that you have that virtual point set to an 'Alternate' change type. This means every poll period of the virtual data source it will flip from true to false or the other way around. Have you disabled polling on the virtual data source that owns DP_Ai1?
I would look at your events to try to figure out what is going on if it isn't a misunderstanding of the change type. I would expect a message about the SetPointWorkItem failing if you see all the events raised / returned that you expect to.
If on the other hand you don't see all the high limit events that you were expecting on DP_i1, then I'll wonder what version of Mango you are using, and I'll want to see the settings on the high limit detector.