I also love this ep to bits. Exactly for all the reasons you've put out there: the shift in tone, the huge strides Team TARDIS have clearly made together, and all that implied backstory that ficcers have been filling in ever since.

I vaguely recall reading somewhere that "officially" Jack put the Hub in lockdown to avoid disrupting the timeline, but it would've been nice to actually *see* that ep. They do enough flashbacky stuff, it could've been worked in, and it's a shame they didn't.