I would be totally freaked out if all that happened to me your doing a great job!

can you post a picture of the one you cracked open or did you "get rid of it"?
For the chick with "veiny"yolk is it coming from its vent or its belly button?
I don't want to tell you to do the wrong thing but if it was my chick I would bring it inside cause it
did not finish absorbing the yolk it needs to be kept VERY clean you might even put a band aid over it. have you ever heard of "duck in a cup"?If you have an incubator I would do the duck in a cup with your chick until it finishes absorbing the yolk.If you don't have an incubator I would keep it as warm as you can(as close to 100 Fahrenheit)
and do the duck in a cup.
I don't know if you should disinfect it because whatever you disinfect it with should end up inside the chick
I'm sorry but I don't know exactly what to do but I think this is what I would do if that happened to me, also could you post a picture of your chick ( with the yolk issue)
I know
@Sally Sunshine wrote an article about what can happen if you don't treat it
https://www.backyardchickens.com/articles/mushy-chick-disease-yolk sack-infection-omphalitis.64686/
I don't want to upset you so if you get grossed out a lot I wouldn't read this and its not what you want to be reading when you want your chick to get better which I do!
@Pyxis any help?
sorry this doesn't help very much and sorry it took so long to write!