Well, the viaduct is still in place, and the station building still exists (albeit currently on the mid Hants). How far is Axminster station from the main road ? I wonder whether there would be scope for park and ride along the lines of St Erth - St Ives.
The station is now a doctor's surgery and the station was miles above the beach (I live nearby). So the end of the line would be even further back at the top of the hill. The gradients down to the beach are about 1 in 4 (if not steeper in places). At about the same distance above the town, on the Charmouth side, they already run a park and ride in summer. From a road access point of view this is a better location, as there is direct and safer access off the A35.
The whole idea of the Lyme branch, as a P and R, is thus a complete nonsense, I am afraid. AXM station is about a mile north from the A35 and there is no spare parking there.
The only possible plan would be to run a line from south beyond the A35 junction, otherwise you need a bridge over the A35. Then relatively plain sailing, as these restoration projects go, but then I think Cannington viaduct is bound to be dodgy. It is fundamentally weak. A cycle track is a much more attractive idea however, and could be very popular.