RE: Planning for a single BOM
This is possible under very specific circumstances only.
If using production, set all prod. Order items to Reordering policy = Order and manufacturing policy = Make to order.
When you create a production order for the parent item, all the sub-assemblies are created as production order lines within the same production order.
Child lines are indented under parent items.
If you use order planning rather than the planning worksheet, you can see supply gaps for all assemblies / materials for that specific order. However order planning has restricted planning capabilities because it behaves in an order by order way. It seems that is what you desire though.