How can I insert an attribute in a certain position rather than as the last attribute?

Make a case table for the collection. Click once on the attribute just to the right of the new one you want to make. Then choose New Attribute from the Data menu. The new attribute will be just to the left of the one you had selected.

Also, you can change attribute order by dragging attribute names in a case table (but this doesn't affect the order in which they appear in subsequent case tables or in the inspector).