Welcome to the forums!
I'm sorry to hear your hen flew away...
Regarding the vein you are seeing on the crop, veins can be visible when the chicks are young and not fully feathered yet, and yes air bubbles can also make the veins more visible.
Their skin is also see through when very young and as they get older, the skin will get thicker in consistency and will darken out a bit. If the chick is bright eyed and active for its age, is digesting the food well, I don't see a reason for you to worry.
As your chicks are approaching weaning age, besides the hand feedings, you can also begin by introducing new foods into their diet. You can place a dish with some egg food, this is highly nutritious for growing chicks and it's soft and easy for them to eat. Seeds easy for chicks to practice dehusking are flax seed, spray millet, Niger seed and canary seed. You can also place some soft leafy veggies for the chicks.
My chicks will usually start to get more interested in cracking seed, eating egg food and nibbling on a piece of veggie by the time they are 23 - 24 days old, but these are parent raised chicks, so your hand fed babies may take a while longer in trying the new foods.