Re: Low Brood Count
Young, unmated queens (virgins), are very quick around the comb, noticeably more so than a mated queen.
I can't say for sure without seeing the hive, but it sounds like what's happened is for one reason or another, probably just under 3 weeks ago, your hive has gone queenless. We know it was just under 3 weeks ago because you have the last few larvae still unhatched. When the hive went queenless they would have raised a new queen. She will have just hatched in the last few days (queen larvae develop faster than workers) and has not mated yet, that will be the one you've seen. Are there drones in your hives yet? There will need to be drones flying in the area for your new queen to mate successfully.
About your photo, that is not the queen cell the bees used to raise your queen, it is just an empty cup that most hives have a few of dotted around the hive, just incase the bees ever want to swarm or supersede.
"We don't need no education" (Pink Floyd) - Yes you do, you just used a double negative.