"Nice post"
Thank you.

"reducing the interest charge from 80 to 28 or by 58 million"
s/b "reducing the annual interest expense from 80 to 28 or by 52 million"

"the Imperial treasury issued new debt"
No, other than the initial 40 million francs of new medium-term notes, the service of the Duthc debt was accomplished by open market operations. In the response above to Digby Smith, I made the very rough estimate that perhaps 10 million francs per year of such transactions were effected for 1810-1814.

"seizures and customs were used to retire Dutch debt instruments"
Actually, it appears that about 100 million francs of receipts of this type were obtained. But, the finds were not used for debt service, but to re-invigorate teh coastal defense system in support ofthe trade blockade. Mollein is very dry in his style, but I sense a note of regret that he did not have these funds put to treasury uses.

"Say, this meant they "saved" half of the 58 million (or 29). Then the "administrative burden" of a separate Dutch army and navy "saved", say, 4 million.
Thus, the Imperial treasury needed to collect 57 million (29 interest and 28 army and navy)."
Recall that from the original 80 million per year of Dutch interest expense, part was merely abrogated. The amount is not specified by Mollein, but perhaps 30 million francs per year would be correct. So I think we are looking at 20 million francs per year that would "migrate" to the French Imperial exchequer, actually viewed as a reduction in income from rentes held in treasury stock, not a new expense. However, given the plethora of issues and the discount on Dutch paper, I think that the coverage was effected at the cost of only 10 million francs per year in lost income, as noted above.
I would think that any savings from "economy of scale" in the militay would be lost in the increased committments requested in preparation for 1812, and then for rebuilding in 1813.

"what was the Imperial source of income to cover these 57 million of expenses?"
Without regard to the figure of such "Dutch" expenss under-taken by the Empire (I provided a sumary estimate in another post), the source of income would be the rest of the Empire. Hence we see Holland as a net "cash user", not a source of cash for the Empire.

"Do you know who "lost" on this element of the "reorganization"?"
Not specifically. Some pensioners and "civil list" recipients, I am sure. But mostly, I think it was mere investors in sovereign debt issues. I am really surprised it took them so long. I would have been looking for abrogation after every change in government !
Didn't Ecuador just do this last week?

"Note: effective interest rates estimated at 10% and 7%, respectively."
Yes, on average, as far as I can tell. Do you also do this for a living, dear colleague ?
But, the Dutch obligations were a really mixed bag, so I am sure that the Imperial Treasury took out the highest rates first, and left the lower effective rate items for actual payment by the Dutch.

Nice post for you, too !