Sorry to hear about the loss of your calf; it is always hard to loose a calf but more frustrating when it appeared to be alive during the delivery.
The way to turn this bad thing into a good thing is to learn from it so it does not happen again.
I don't know if you had the cow close enough to a chute where you could have assisted her. When we calve, we keep the cows close and we are pretty quick to assist. Assistance to the cow, especially with a large calf and a difficult delivery is so worthwhile in saving the calf and the cow stress. The quicker the calf gets out once the feet are through the pelvis, the cervix is wide open and the head is there will pay you back in time it takes the cow to rebreed and also in how the cow feels about the calf after she has it.
From your description of what happened, an hour is way too long for the head to appear after the feet are out. If a cow is actively pushing for 30 minutes with feet and head out, that is for sure too long. But the time the cow spent with the calf hanging partially out was probably the most critical. There is a point during the calf's delivery when the area where the umbilical cord attaches onto the calf's belly passes through the pelvis. When the calf is large and the pelvis is very full the cord is compressed against the pelvis and so oxygen is cut off. But if the delivery of the calf stops at this point....with the umbilical cord compressed by the pelvis and the calf does not continue to deliver to the point that it can fill its' lungs with oxygen and breath, the calf simply suffocates. Can't get oxygen through the cord and is not far enough delivered where it can expand its' lungs to take in enough air to survive. A calf really needs to deliver to the point that the ribs are all out to be able to breathe, as happens in a hip lock.
I would not criticize the cow for not delivering faster; we have calved thousands of cows. Sometimes when a calf is very large for a cow, labor does not kick into high gear very quickly; the uterus is over-expanded and so can be lethargic. Your cow went through a very long and difficult delivery; she does feel pain. We have found that after a particularly painful delivery, a cow will walk away from her calf, especially a heifer will tend to do this; the pain overrides the instinct to mother. However, we find that usually given 30 minutes or so, the pain will subside enough that the cow will mother the calf. It is best to have the cow and calf in a small area for the bonding process to take place.
I would not criticize the cow for walking away either, especially because she did raise a calf the year before as a heifer.
That is how I see it.