You are correct that we cannot change the "Selected Event" action to attach a custom action.
Now, the reason why BOTH views went to the Summary View by default is because they were looking for a Detail View to navigate to within that same datasource (your slice). You have two, the system generated one and then your custom view "Summary". AppSheet uses the custom view by default, in your case Summary. If there is more than one custom DEtail View, it will pick the first one in the list.
This is a known, long-standing problem. The way developers workaround it, is to create a Slice copy. In your case, I would recommend dedicated Slice copy for the calendar, then you will have be able to create the Detail view for that Slice in any way you wish to compliment the Calendar.
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