Rob, when you move here you’ll probably be pleasantly surprised to find how transparent these things are in France. The French government website sets out very clearly what the requirements are for any given situation. Residency is dealt with at departmental level and there are sometimes slight differences in local interpretation, some prefectures will insist on this but not that and others will insist on that but not this. There are gripes about this, but by and large the process happens as it says on the tin. Immigration on the other hand is dealt with by consulates and OFII and again, they all follow the same rules. Basically the way France sees it, there are rights and responsibilities on both sides. If you meet your obligations, you will be granted the corresponding rights, if you don’t you won’t, simple as that. There isn’t really a role in the process for a third party. If you meet the criteria you normally won’t have any difficulty in producing the paperwork required to prove it. If you don’t meet the criteria then no third party will be able to arrange for yçu to be granted a right you’re not entitled to, French immigration runs a tight shop and it doesn’t leave back doors open.
All that just to basically agree with Stella - save your money, wait and see what hoops they put up, and then jump through them with goodwill.
Until you live here, there is nothing you can do to establish any kind of residency rights.