I hate plot holes, and LARGE inconsistencies, even when they're minor to the story, so I'd love some help rationalizing the beginning of Lessons. I was pretty sure head level officials/department members (heads) had to report to the captain as soon as coming aboard (or ASAP). Why handn't Picard met Darren? She was new, but not just gotten there, nor was she (apparently) doing anything that could be used as an excuse (emergency). The whole idea that the Stellar Cartography Dpt could ask/order ship wide blackout of comms, (library) computers, replicators, etc, with no one ever passing it by the captain before, seem ludicrous.
The crew seems to be easing back into duty after some interrupted R&R in "Starship Mine". Of course Picard gets it the worst, so he more than anybody is taking it easy and keeping things casual. The Enterprise seems to be free and clear of any danger, maybe some personnel transfers are being made (like Darren). When Picard has to be on duty he wears his Captain's Variant uniform, suggesting a more relaxed form of order. He's taking time for himself and doing things that he wants to do, after all, he didn't get to go on that horse ride. I would say if the Captain is taking some much needed time off aboard his ship, and the Enterprise is guaranteed to be in friendly space, these are circumstances where a certain flexibility is allowed in the Enterprise's command structure, where such a huge diversion of the ship's power resources could be authorized by somebody other than the Captain. A few scenes after Picard meets Darren, Riker tells Picard on the bridge that "Commander Darren has already requested extra time on the Main Sensor Array" since the ship is on its way to the Borgalas Nebula, and he says it somewhat flusteredly too, suggesting that perhaps Darren requested the power diversion from Riker the night before, and he gave his approval begrudgingly.
Somewhat thin... but okay yeah... I could take some of that. Darren was implied to have been there for some time (long enough to find the acoustic sweet spot for example)... they've might've even said weeks at some point. I like the "down time" relaxed thing... Maybe despite protocol, Picard agreed to let Riker (and others) more leeway on that level decision ("I'm F'N tired Number One... you take care of sh!t for a while... I will have anyone who bothers me scrubbing plasma ports for the rest of their career!"). I read the bit "Commander Darren has already requested extra time on the Main Sensor Array" as more time than she was scheduled (like all relevant science departments would be)... but I could see it being as you say... I am definitely open to thin patches (over "just because")
Stellar cartography could be a subdepartment of the science department, so she probably reports directly to head of science, but Picard should certainly know she exists. In other episodes he's been shown to need to approve all transfers aboard the Enterprise. The would have seen each other and he'd know who she was, but might not have interacted face to face in a context they can really know each other beyond formalities. Riker in other episodes has been shown to run day to day operations on the ship, so he might not bring requests to the captain unless they are out of the norm. Part of Riker's job is to keep the small minutia off Picard's desk.
I dig it... I specially like that she's low enough not to merit necessarily a face to face... though stellar cartography... which is major to navigation, would seem important enough to want to get to know... but hey, that's up to him right so yeah. But comms black out seems major without captain's stamp, what if there WAS an emergency? I thought that was all of Riker's job on a day to day, obviously command as well... but I mean not like Spock was also science officer (yes I realize that was a smaller ship, but that also implies "most" of Rikers job is administrative).