Marcel,
about your many detailed *technical* questions about the history of character properties, I am afraid I have no specific recollection.
French is not the only language that uses a space to group figures. In fact, I grew up with thousands separators being spaces, but in much of the existing publications or documents there was certainly a full (ordinary) space being used. Not surprisingly, because in those years documents were typewritten and even many books were simply reproduced from typescript.
When it comes to figures, there are two different types of spaces.
One is a space that has the same width a digit and is used in the layout of lists. For example, if you have a leading currency symbol, you may want to have that lined up on the left and leave the digits representing the amounts "ragged". You would fill the intervening spaces with this "lining" space character and everything lines up.
In lists like that, you can get away with not using a narrow thousands separator, because the overall context of the list indicates which digits belong together and form a number. Having a narrow space may still look nicer, but complicates the space fill between the symbol and the digits.
Now for numbers in running text using an ordinary space has multiple drawbacks. It's definitely less readable and, in digital representation, if you use 0020 you don't communicate that this is part of a single number that's best not broken across lines.
The problem Unicode had is that it did not properly understand which of the two types of "numeric" spaces was represented by "figure space". (I remember that we had discussions on that during the early years, but that they were not really resolved and that we moved on to other issues, of which many were demanding attention).
If you want to do the right thing you need:
(1) have a solution that works as intended for ALL language using some form of blank as a thousands separator - solving only the French issue is not enough. We should not do this a language at a time. Do you have colleagues in Germany and other countries that can confirm whether their practice matches the French usage in all details, or whether there are differences? (Including differently acceptability of fallback renderings...).
(2) have a solution that works for lining figures as well as separators.
(3) have a solution that understands ALL uses of spaces that are narrower than normal space. Once a character exists in Unicode, people will use it on the basis of "closest fit" to make it do (approximately) what they want. Your proposal needs to address any issues that would be caused by reinterpreting a character more narrowly that it has been used. Only by comprehensively identifying ALL uses of comparable spaces in various languages and scripts, you can hope to develop a solution that doesn't simply break all non-French text in favor of supporting French typography.
Perhaps you see why this issue has languished for so long: getting it right is not a simple matter.
A./
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