I hope your neice and her baby are doing well.
It is a little difficult to sort things out based on the limited information posted here. It sounds as though there was some uncertainty as to her due date. That is not uncommon, as sometimes women do not recall the exact date of their LMP, or their size does not match their estimated gestation. U/S done during pregnancy has only limited value in pinpointing a due date, so even if done, it may not be particularly helpful.
The condition of the umbilical cord and meconium stained amniotic fluid could be related to a number of medical or obstetrical conditions, so I won't speculate as to their cause. They are consistent with your description of fetal distress. It is a credit to your neice's caregivers that baby arrived safely.
You describe the baby's arms and legs "flailing" after delivery. That is not of concern. Newborn infants are used to the dark, quiet, cramped conditions of the uterus, and are most comfortable when they are in a curled, flexed position. After delivery, when infants are placed on the warmer or examining table, they become startled by the bright lights, sounds, relatively cool air, and loss of their "contained" environment. They often react by screaming bloody murder, and flailing their arms and legs. Their immature nervous systems are over-burdened by all the stimulation. This is not a sign of illness, but a normal response. And a good, strong cry helps to clear the lungs of retained amniotic fluid (once the meconium has been cleared from the baby's airway, of course.)
You also describe some difficulties with breastfeeding. As frustrating as this is, it is not unusual either. Contrary to popular belief, breastfeeding IS NOT instinctive for baby or mother. It is a learned behavior. And like all learned behaviors, some moms and babies catch on more quickly than others. Sleepiness, lack of interest, poor positioning of the tongue, inconsistent sucking are all common problems in the early days. I sincerely hope that your neice receives assistance from the mother-baby staff or a lactation consultant to overcome these minor difficulties. None of them should present a true barrier to breastfeeding. It sounds as if the baby's oral intake has been very limited. That must be addressed to prevent dehydration and excessive weight loss. This may be accomplished by frequent breastfeeding, or by some type of supplementation as recommended by the nurses and doctors caring for your neice and her baby. Adequate urine output (at least 4-6 wet diapers per day) is vital to indicate adequate fluid intake. The baby has already passed meconium, so the lack of a bowel movement for the first day or two should not be of huge concern. Once regular feeding is established, bowel movements should follow

.
Thanks for your interest and concern. Best wishes to your family!