the upper box ... It has capped worker
Either the colony is queenright or it has a DLQ, but it can't have both.
QC in both boxes plus at least one open one
These could be from the swarming period and may be empty. For mysterious reasons, bees often reseal emerged QC cells. See if you can flip the lid with a hive tool. Is the open QC viable? It may be without larvae and dried out.
it's a genuine queen and it hatches
If it does, the bees may swarm with the current queen, once in the super but now down in the BB.
1 If the colony is producing worker brood and you've shaken all the bees into the BB, then it's queenright and you won't see QCs. Leave 3-4 weeks before checking for eggs, but only if a colony is re-queening.
I'm sticking to one part of your info (that worker brood is present) and conclude that as worker brood cannot be laid by a DLQ, the colony is in fact, queenright.
2 Two factors will guide your decision: first, it takes two DN frames or the equivalent to feed a colony for a week, supposing it is unable to fly to forage. Secondly, the main flow is on. Without knowing colony strength, you alone must decide whether to leave supers on or take off.
You might take off the full super, extract and return it the same day for a refill.
pay more attention at around 2-3 years
Suggest you re-calibrate your beekeeping routine and agree with yourself to check bees every 7 days from March - July (depending on the season). Do not delay a check because of rain or other minor distractions.
AS takes 5 minutes. Practice the routine with empty boxes.
Q would have been 3-4 years old.
Swarming to replace her is almost guaranteed.
keeping the B1.5 boxes unstuck.
Simplicity in beekeeping is a great asset and brood & half is not one, because frame management can only go one way, and because laying space is clearly not enough. Go double brood and feel the freedom!
Edit: if the QC is viable you could put in a split board and separate the BBs. If the QCs are dud or fail to mate, re-unite later.
Split board: find a 460mm square piece of ply, pin & glue 8mm stripwood to both rims, and cut a small entrance in one top rim.