Patrick,
You have identified the issue with point 2 (and the example)!
Either you can have pricing at the Main item or Sub item level, not at both levels. Thus choose to deactivate pricing in any one of the two levels.
If pricing is activated at sub item level, then taking your example -
the sub-items are USD100, USD60, USD40 respectively,
the Main-item value is USD 0 (actually pricing is deactivated)
the document header Net Value becomes USD200
