Good question OP. Unfortunately, since this is a forum for travelers rather than serious language learners, few people here know any of these. We do have one US-born French teacher who posts here, Alexander, so hopefully he'll see your question. I know he did some of his studies in the Loire Valley.
I looked at your three and I do have a tiny bit of info. Some people have posted here about the one in Villefranche. Those were not people trying for a DELF score, but people who chose this to relax: food, wine, culture and language. I think they remain in French during excursions and meals, but I'm not an expert. What level of conversations can they have at the dinner table with one teacher and several learners? Another drawback is the area is filled with foreigners, many, many second homes and the lingua-franca is English.
The one is Tours is where Alexander went, and he raves about it. Several American universities have their junior year programs in Tours. This appears to be a serious program, but we need Alexander to write something. The Tours accent is considered the most standard in France, the one TV and radio personalities are taught. Good if you have an oral DELF component.
The REMPART program is a work program, a good way to immerse. It's mostly young volunteers, some from other European countries, which means that the lingua-franca could be English, again. But you could insist on lodging with a local French family and be included in their activities, be on a site (chantier) with many participants. Today accents all over France are more standardized, but regional accents do exist, and they will retard your progress as you have to re-filter the new sounds of old words and piece it back together. Drawback: no language program, only work and you could be building a stone wall all alone for a few weeks. We see these programs on TV sometimes.
