I found the original article and several postings here to be very helpful and gave me the needed support to intervene.
My intervention story: On my second hatching experince, I was hoping to avoid intervention. Sadly, by the end of day 21 with four eggs pipped and somewhat zipped and no progress, I discovered all four dead in the shell. So after that discovery, I subsequently helped two SFH out of their shells. I also helped 3 EOs that had pipped internally and were chirping in the shell but not making any progress on Day 22. So I carefully used a drill bit to scratch a hole in the large end of the shell and proceeded to carefully remove bits of the shell and membrane to expose the chicks. There was some blood from the membrane piercing. I allowed each chick to rest for a while and then opened the shells further. Eventually, each of the chicks were removed from the shells with the yolk fully absorbed. Two navels were mostly completed but I applied some antibiotic to them anyway. After a few more hours of rest, I gave each of the EOs a bath to removed any residual membrane and dried blood.
While this was somewhat nerve wracking, I am happy to say that all 5 of these chicks are doing well running around and eating like crazy. My midwifery efforts appear to have been successful.