It isn’t “extra” mezonos; it is a kenas. And a kenas renders a Get to be a Get Me’usa. Any payment amount above the halachic amount required for mezonos is a kenas. Even the supporters of the prenup admit this. (I asked this of a major prenup supporter whose been a dayan on the BDA and he expressly stated as such.) If the payment amount was $500/day instead of $150/day (and the terms of the prenup were otherwise exactly the same) even R. Willig would admit it is a kenas, and it renders the prenup halachicly invalid and causes a Get Me’usa. But the supporters are being deceitful in claiming $150+/day, $55,000+/year, [plus her keeping her own salary] is not higher than a mezonos amount (as I clearly demonstrated in previous posts on this thread.)
Regarding the lack of reciprocity, her not receiving $150+/day is not much of penalty and certainly isn’t reciprocal to his having to pay her.
Regarding the start date, she can walk out and give him immediate notice he must begin paying her.
The Nachalas Shivah is not designed to force or pressure the husband to give a Get and was not for divorcing cases. It is the pressure of a kenas that renders it a Get Me’usa and potentially causes mamzeirim c’v, an inapplicable issue in the Nachalas Shivah case.
Her keeping her salary while he is paying her mezonos is halachicly effectively an additional kenas, since that salary is his property under halacha while he is providing mezonos.
“So, you’d be happy if the prenup was a smaller figure, or depended on income, or on what standard of living the wife was used to?”
Yes. If the amount were the same amount as Shulchan Aruch specifies a husband is obligated to provide mezonos and the amount was only payable in halachic situations when he is obligated to pay her mezonos, then ???”? it would be halachicly acceptable. Anything above that amount, or applicable in additional situations, renders it a kenas.