Boomerang swarm

This was a hive I split last week.  When I had looked at it then I found two sealed queen cells, three unsealed but charged cells and the yellow-marked queen.  I put the queen and two frames of sealed brood in a nuc and made a note to go back after five days to reduce the queen cells to one.  However, having been away for a few days I got a bit mixed up with the timing.  My neighbour Gill told me last night that the bees had "escaped" again yesterday but had flown around her garden without settling like they usually do (poor Gill is used to my swarms). After that she didn't see what had happened to them.

This morning I inspected the hive and found an opened queen cell (!), three sealed cells and two charged cells. As I did so I could hear a queen piping and I hoped, and assumed, it was coming from the one queen cell I had decided to leave.  I checked the ones I removed and they only contained early white pupae.  An hour or so later I was looking through the last of six hives when I heard a roar and saw a swarm emerging from the first hive.  The bees flew over the fence into Gill's garden and as I watched they swirled around but didn't settle or coalesce. After a few minutes I could see they were flying back over the fence and within a few more minutes they had started to settle on the front of their own hive.

There were also bees all over the plants in front of the hive and on the ground.

The video shows them arriving back.

Despite the roof being too hot for me to touch in the sunshine the bees were walking and standing on it.  Here is the front row of bees taking the strain.

Within minutes lots of bees were fanning at the entrance and those bees nearest started to walk down to the entrance.

Within half an hour of landing they were all back in the hive.

My curiosity got the better of me and I decided to look in again this afternoon.  I noticed there were a lot of bees on top of the frame with the one remaining queen cell (marked with a red drawing pin).

And the queen cell was still sealed.  And I could still hear the piping, coming from somewhere on the same frame.  After searching I eventually spotted the new queen.  She was small and mobile.  I tried very hard to video her but every time she disappeared under a scrum of bees.  I did manage to record her - listen carefully at the start of this clip - but I don't think she is in the picture.

I did manage to get a slightly blurry photo - she is right in the centre.

All this was quite difficult, balancing the frame in one hand and videoing with the phone in the other in bright sunshine.  I didn't want to lose the queen so I removed the last queen cell and put her frame back in the hive and closed up.

I then had a look at the queen cell I had removed.  The bee within it was very obviously dead and had been for some time.  There was also a white blob which may have been a larva.  If the dead bee was a queen I assume she had been killed by the first queen to emerge, presumably yesterday or the day before.  However, thinking about it, the dead bee was small and looked as though it had been dead for more than a day or two - could it have been a mummified worker sealed up with the queen larva by mistake?

All this is something I have not seen before and several other questions come to mind.

1.   Did the bees return to the hive after swarming yesterday as they did today, and if so why?  If they were lead by the new queen, at that stage there was a viable sealed queen cell still in the hive.

2.   Were the bees programmed to swarm with the new queen and only then "realised" at the last moment that there were no viable queen cells left behind after I removed all but one (and the one was a dud)?

3.   Was the piping warning the bees that the virgin queen was planning to lead a swarm?

Any comments will be very much appreciated.