Just something like a clarification: You can only Adjoin to something that doesn’t have any Merges left it needs to do. Effectively, this means that you can only Adjoin to XPs. You don’t have to necessarily wait until the very end of the derivation to Adjoin — you just have to do all of the Merges first for the projections of any given head. That is, you can Merge the V and its object, forming a VP, and if there are no more [uN] features on the V, you can then Adjoin something to the VP before continuing on to Merge the v and VP (to satisfy the Hierarchy of Projections).

What you can’t do is, say, Merge “to Mary” and “introduce” (which will check off one of two [uN] features it has), then Adjoin something to the result, and then Merge “John” to the result to check off the other [uN] feature. You have to check both of those [uN] first with Merge, before you Adjoin.