Yes, good to see you back online.
I think the most straightforward way for you appears to be for the first local authority that you will be visiting in France to issue the business invitation letter. The ideal situation would be if they could explain that you will be travelling to the Schengen area frequently over XX period for XX purpose to justify the need for a multiple entry visa that is valid for the period you require.
I was in a somewhat similar situation 2 years ago, needing to travel across several Schengen states over the period of a year for a programme of work. Getting the invitation letter in time with the relevant information proved to be somewhat of a challenge for me as well. The main reason being, the inviting party not being familiar with immigration matters. I ended up drafting a lot of the letters myself after reading the requirements posted on the embassy websites and getting some information from the third party that deals with immigration matters for the company I work at. The letters should be printed on official company letterheads and you should have the original sent to you by post/fax to submit with your application.
Just to give you a sense of how they appear to decide on the validity period of the visa they issue, it does appear to be closely linked to the recent Schengen visas that you've had and whether you've adhered to the visa conditions - i.e. no overstay etc... What happened in my case was, I requested for a multiple entry visa valid for 1 year when I applied for business travel (German Consulate). They however only issued me one that was valid for 3 months. I had one single trip Schengen visa in my passport at the time I applied. I had to apply for another Schengen visa, before I was granted a circulation visa - valid for 2 years by that time even though I didn't need it. I ultimately had 3 Schengen visas issued to me over the span of 1 year.
My final Schengen visa was issued by the French consulate and was quite straightforward. You can find the complete list of documents required on their TLS contact website.
https://fr.tlscontact.com/gb/LON/page.p ... ness_visit
Hopefully that will give you what you need. Just post back, if you have any further questions.