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Was helping another beekeeper today and we discovered that one of the hives had 3 swarm cells on it in various stages of development. Two of the cells were capped and one had a developing larva in it. She is using 8 frame equipment and the swarm cells were on a medium frame. We found that the queen was down in the deep box. We then added another undrawn medium super on top of the deep box and placed a queen excluder on top of that box. Then we placed the medium super with the 3 swarm cells on top of the excluder and put another empty undrawn medium on top of that. The swarm cell medium super had all of what I thought was the right items in it - capped brood, open larva and stores. Since we did not have another 8 frame hive top to do a traditional split we decided this was the way to go. She uses Boardman Feeders and has a division feeder. I advised here to use that in the top super. We are both out of town till the 1st week in August, when we plan on seeing if she has a laying queen in the supers above the excluder. Should or could we do something different that the above actions?

Maybe I'm missing something, but how is the new queen supposed to get out to get mated and then get back in?

Oh yeah that would help! Might need to make a top for an 8 frame so we can do a normal spilt or is there another way is this case? An Imirie Shim?

Two queens can sting through the openings in a queen excluder.
What you've done will not prevent swarming.
The current queen will still leave with the swarm.
If you want to make a split use two hives, moving the current queen to the new location. Additionally, in most cases of swarming, bees will construct more than three swarm cells. It is possible they are superceding the current queen rather than planning to swarm. Include at least one frame with eggs with the queen in case she is failing.

Or we could remove the excluder and let nature do the rest?

If you choose not to split into 2 hives, I would remove the queen excluder, return the hive to its original configuration, place one of the medium supers with foundation in the top position.
This will give them extra space to prevent overcrowding.
Overcrowding is the main cause of swarming.
Check the hive when you return to confirm that the hive is queenright and proceed from there.