I agree that the first issues were lack of oxygen and soil too compact. I know some growers have had good grows with slightly above 6.2 PH but you should try and drop that down to 6 or 5.9 and you could get better results? Just what I have read. I had similiar issue as you and making the ph 5.8 fixed it right up the same day as the watering and I am in 100% miracle grow soil
( Planted them before I read up ) but both plants are thriving as I think the other issue was how much you were watering them at once. Over watering is hard to do with adult plants but seedlings like yours maybe tuning down the amount and increasing the frequency, along with mixing up the top 1/2 inch of soil to aerate it better is good for 100% soil plants. Water fresh water every time and you will never have to aerate watering solution unless of course you have a permanent water feed system.
The coco-coir you have is same stuff I got. The water drainage is 100% improved from what those guys were in before and it is HARD to over water plants in a hydro medium. You really have to be drowning them. Most hydro mediums have zero nutes. be careful not to have your coco coir not mixed and compacted, this can also cause some soil-like compact conditions. Try mixing anything that is stable and not readily water soluable like perlite and mix that with your coir, will work wonders! I am going to try 2 different sets myself, I have aquarium safe rocks that I will be trying as a mix with coir and also perlite, to see which does better. Would like to hear from your results as well!
Also I think I read someone said to not be nuteing soil plants at seedling stage, this is generally a great idea unless you want some unecessary worrying and some burn as the light and the nutes in the soil will feed that sucker for a good few with just normal ph adjusted, dechlorinated tap water.
As for random plants going, it could be genetics. With a bad start like they had easily a plant can be weak enough genetically to only grow to a certain point and die or just die right away. Transplants are alot of stress too especially from soil to coir if you removed all of the soil from the plant before the transplant its like taking a person from a thriving market and putting them into poverty, they arent used to it! Deaths can and will occur!