the location looks too moist to me, too.
swarming: at least for the european bees I know in our climate it is quite normal for a hive to swarm several times. first, the large swarm with the old mom. after that several smaller ones with unmated queens. there is nothing like "holding the queens in their cells" around here. never heard of that. they co-exist and then take flight with some bees.
so....
maybe they didn`t get enough room so you get so much swarming tendencies? meaning: if the brood-nest clogs up with honey (no supers, supers with no comb, maybe even without foundation?), they can`t breed any more and ... leave.
as for swarming and a dwindling hive: if a large "pre"-swarm and several small "after"-swarms go off, it should have been one heck of a hive. there will be plenty of brood in the old box. no reason to dwindle at all!
if they dwindle, they got a problem. probably: varroa.
when a swarm goes off, or several, let`s say, half the number of adult bees just for math`s sake, they take appr. 5% of the varroa along. 90% is in the brood. so all of a sudden, the ratio varroa to bees increases quite a bit. if the hives haven`t been treated right the fall and winter before, they got too much now and if you don`t really control them, they will dwindle because sick bees don`t breed good bees. so it crashes evtl.
get another source for your bees, keep an eye on varroa (alcohol wash, not your natural eye) and treat fittingly to time of year and condition of the hive.
honey was too moist. harvested too early or stored not fittingly. one or two days in the comb at your lake-site before extracting or letting it sit around in a strainer will do it, too. I got a moist environment, too. I use an air-dehydrater now, cause I cant extract that fast.
