Do you know if:
- there are bicycle tracks one or both sides and along/adjacent to Sèvre Niortaise between Magné and Coulon?
- there is any fence blocking tracks at some point?
- the surface is asphalt or dust trails?
- one side is better for riding?
The recommended cycling path between Magné and Coulon is on the road: the roads seem small and not busy, but that's far from the Sèvre niortaise river. Few parts are still completely protected from car traffic.
There is still a path on the right river bank of the Sèvre niortaise, in the so-called Marais Pin. It is however not very accessible, with fragile bridges and it can be pretty muddy. There seems to be a plan to build an actual cycling path that dates from 2014, but there is no report of it being actually available (the local administration in charge of building it does not list this itinerary on its website). The article stresses the negative impact of a cycling path on the biodiversity along the river and stresses how wild this area is so far.
On openstreetmap, the right river bank path is explicitly marked as not accessible to cars but nothing else is mentioned, so it seems practicable. There is also a path on the left bank, it might be more accessible, even to cars (the article mentions it too).
If you decide to go ride on the right river bank, it will probably not be very easy to access, so while there might not be an actual fence on the way, there will likely be narrow tracks sometimes, or obstacles on the track, or narrow and fragile bridges. Do not expect dust trails but rather a muddy, soft trail, especially between fall and spring.
Finally I got a chance this June to ride this route and found it very delightful in the morning. Although from Magne to Coulon has no proper compacted bike trails but both sides of the river there are navigable bike tracks suitable for VTT (mountain bike) as these are only traces of locals driving vehicles on natural vegetation for finding proper fishing spots by the river. I suspect most bike riders reaching Magne switch to highway because they are riding road bike and not VTT. I tried to upload photos but uploader complained pic size larger than something.