I have been growing in Coco for years and I never drop my pH below 5.8 and I tend to veg at 6.0-6.2 with optimal results . It is best to stay on the high side verses the lower side especially when using a cheap test kit .
Allowing Coco to become dry can bring on deficiencies very fast and you will need to use cal mag at 5 ml to a gallon to avoid the calcium issues your seeing and do feed it more as it is not a seedling