On the face of it, for 73 in a 30, the question is not whether she will be banned, but how long she will be banned for. Under the current Magistrates' sentencing guidelines, Band C (7-56 day ban OR 6 points) applies to 51+ in a 30 limit. Under the previous guidelines, the equivalent band was for 51-60, with anything over that beyond beyond the guidelines. The current guidelines also state that for speeds grossly in excess of the limit, a ban exceeding 56 days should be considered.
Your post is one of the best formatted posts we have seen on this forum. If it had contained any useful information, it would have been easy to access.
The first question would generally be whether there is a viable defence. You indicate that there was an issue with the signage. Is there a compliant system of street lighting (lamp posts no more than 183m apart)? What signs were there? How were the 50mph signs on the other side of the dual carriageway visible? Can you evidence what was there?
As regards mitigation if there is no viable defence, the hive mind is generally of the view that absent a genuine emergency, there is very little that can be effective mitigation for speeding. I am generally of the view that the hive mind often does not fully understand the question.
The issue is that sentencing for speeding is generally very prescriptive - paint by numbers, presumably stemming largely from an assumption that the greater the speed above the prevailing limit, the greater the danger posed, and to a lesser extent, the greater the level of criminality or disregard for the law.
If she has no defence, she has no excuse for breaking the law. Any mitigation should be phrased on the basis that she fully accepts that she broke the law and has no excuse for breaking the law, but respectfully invites to consider whether in light of the following <the mitigation> (which does not excuse what she has done), the true seriousness of her criminality is reflected by the numbers - which taken in a vacuum, 73 in a 30 limit paints an exceptionally frightening picture...
If she is seen to argue that it's ok to speed through roadworks when there invariably are no roadworkers to protect, that will not help her cause. What she needs to get across is that there is a very material difference between the circumstances of her crime and that which 73 in a 30 taken in isolation would suggest.