Here's my take on it...
Get the started pullets from the place you got chicks IF those birds were not the ones that died mysteriously. It may have nothing to do with your flock or conditions but genetics.
Since you know you've had coccidiosis, know that it can scar the intestines, even when treated, that leads to life long malnourishment and wasting. Heat stresses things even more. I have had some wasting deaths after a coccidiosis outbreak, and after treatment, and after apparent healing. Those affected birds were never do wells afterwards likely from intestinal injury and eventually died, though not at the same time.
So my thought is the coccidiosis and your deaths, other than the storm caused, are likely related.
Some birds genetically have more strength to fight off coccidiosis due to stronger immune systems and natural resistance. Back to your chick origination. Can you remember where you got the birds that have not been thriving? Then, order from the other guy if you can.
Then I would isolate the started pullets not so much for your flock's safety (as I agree with
@rebrascora ), you really can't isolate from Marek's unless you've got a bubble tent and chicken hazmat suits...and if the one is 10 minutes away...they've likely been exposed too as it is in the area. So, personally, I wouldn't worry about Marek's.
I would be concerned about the coccidiosis. Those nastly little protozoa are opportunists. They lurk in the intestines growing exponentially with the right environmental conditions. Then they mature in the gut to release thousands of oocyts that are the seeds for the next generation. Your soil likely has high contamination, so litter and ground management will be necessary as well. If able, field rotate. If not that, then shovel out litter and replace with fresh.
Bloody diarrhea only happens with certain strains and only if it attacks the lower intestinal tract. Other strains are more insidious lurking higher in the tract robbing the bird of nutrition.
I would put your started pullets on medicated feed so that they build a slow immunity to the strain on your soil, and putting them on fresh litter in the goat shed. Watch their poo for volume and color. Coccidiosis sometimes simply creates voluminous fluffy poo and that huddled, scruffy look. Heat stress adds to it.
Once your started pullets are stabilized, looking healthy from transition stress, after about 30 days, then I would introduce them to the main flock.
At some point, you simply have to breed for resistance. While meds are very useful, quite honestly they were invented for flocks that were kept in very cramped quarters for high turnover and high profit. Long term sustainability was never the main goal.
As backyard flock owners, we can manipulate and adapt for longer term sustainability, breeding from the stronger ones, using meds and management as necessary, but always with the long term goal in mind.
Buying strong breed lines from an area close by is generally the best idea IF that was not the source of the birds that did not thrive.
My thoughts.
LofMc