I’ve seen this handled various different ways in different units. Before I get into different schemes I’ve seen, I wanted to clarify something from your post:
It seems like you can’t purchase the rank badge (or other restricted items) without Leader Approving it first, since I believe that the item doesn’t appear in the database for purchasing (i.e. Needs Purchasing report → Purchase Order) until it’s Leader Approved, not just Completed.
Back on the topic of how different units handle this, I’ve seen several versions discussed.
Some units create a policy indicating that the only people who should be marking things “Completed” are the same people who are authorized to mark them “Leader Approved”. They will ignore or “unmark” items that don’t comply.
Some have policies that require that, if a scout, a parent, or a scout via a parent account marks an item complete, they need to provide some sort of “traceable” verification in the comments. For example a photo of the relevant signature page, or notes identifying who signed off the requirement and when so that the adult leader marking it “Leader Approved” can back-check the information before approving it in Scoutbook. Similar to the first case, they will ignore or “unmark” items that don’t comply.
I’ve also seen a “hybrid” approach where ranks, awards, and their requirements are Leader Approved by a selected group of ASMs, merit badges are entered and approved by the Advancement Chair upon receipt of the signed blue cards, and nothing is marked Awarded except by the Advancement Chair. The ASMs are responsible for keeping up with their assigned scouts’ advancement data (e.g. checking in periodically for new signatures in handbooks, watching for notice of completed BoRs, etc).
Others, as @Matt.Johnson notes, ignore any interim data altogether. Those groups fall into a couple of general camps, as far as I’ve noticed. Some will only Leader Approve the final rank/badge/award, and never mark the intermediate steps Leader Approved. Others Leader Approve all related requirements, along with the final rank/badge/award approval.
@Matt.Johnson raises an important question. This seems like a unit leadership (whether SM corps or committee) policy decision, since whatever is decided has implications for how the unit operates. It probably shouldn’t just come from a single individual without review by the relevant leadership cadre.
The only exception I would take to @Stephen_Hornak’s remarks is that there is no way to accidentally sign-off a large number of requirements that have a checkmark in the left column of the handbook. It’s very easy, given the Approve All button, to make this mistake in Scoutbook. It’s also very difficult to remedy such a mistake (albeit less difficult than un-signing something on paper…).