What a I've found with assisted hatching, and I've done a few(some I shouldn't have),
is to have your work tools and area all ready to go before removing egg from bator.
I use a hinged arm utility lamp to warm the area and to able to see well, I put it down low to warm area first then raise as...
When to assist is a tough decision...I think the rule of thumb is 48 hours after internal pip. There's a good thread or article, not sure which, about assisted hatching, good tips and tricks in there. Here's two of them, both are good...
A pip not in the air cell can take longer to progress ti zipping,
it's like an internal pip so twice as long to zip.
As long as the bird can breathe, it should be fine.
Yellow just looks like dried membrane...unless there was a puddle of yellow, which could be a broken yolk.