Here my two cents: It seems to me that he top kill flow is going to the path of least resistance, so some of it could be fracing a totally different interval than the pay zone, so the pay zone may never receive cement (and even if it did it probably wouldn’t squeeze off) so they are attempting to achieve good cement in the annulus and inside the casing which will require a balanced plug, which will have to be calculated on the fly based on what sensors they have working. One of the reasons there’s a 30% chance of failure.
Sometimes system technologies don’t progress in lock step. In deepwater the drilling technology has out paced other components of the total system, ie. they use accelerometers, flux gate magnetometers, and gyros to control a motorized eccentric sleeve to point the bit to stay within several feet in a lateral target for many thousand feet. Some of the emerging drilling technologies are drilling with casing and wired drill pipe.
Though it appears there are some production string design questions, the well control system crashed beyond anything they had prepared for, remember our government explaining “who would have thought terrorists would fly into buildings?” Repairing and maintenance on the well control components by ROVs has been impressive, but if they had spent the money they spent lobbying congress to improve ROV technology instead, they would have been further ahead.
Deepwater rigs are run by computers but this demonstrated the remaining possibility of human error on several fronts. The logical thing to me would be to have a back up surface bop with a high pressure riser, and the rig hands should demand this. Fifty year old technology could have prevented this blow out even in the event of not following proper well control procedure.