Carolina,
You are right; unfortunately, the use case of automatical aggregation of sprint members by a common value of the property exceeds the features of MDX implementation. The manual collection of sprints might be the way to go in your case.
You are also right that aggregated members do not inherit the properties of all the children. It is possible to access the members of the aggregated member with the help of the ChildrenSet function. For instance, if you have aggregated all the sprints by the Complete date, you might wish to have the formula like this:
CASE WHEN
[Sprint].CurrentMember.Level.Name="(All)"
THEN
ChildrenSet([Sprint].CurrentMember).Item(0).get('Complete date')
ELSE
[Measures].[Sprint complete date]
END
Your attempt for the workaround to aggregate the measure over sprints having the same end date will not work either, unfortunately. You have the Sprints in your report and iterate over the Sprint dimension in the formula, which confuses the context in the calculation.
So, seems that the manual creation of aggregated members might be the only option for this case.
Kindly,
Janis, eazyBI support