I think your proposal would be ideal for Ilfracombe, Bideford, Torringon and South Molton with the proviso that through London to Barnstaple trains are provided, as well as for Sidmouth, Seaton, Lyme Regis, Chard, Yeovil?and Shaftesbury (and Bude and Launceston assuming Okehampton reopens and has through London Trains.
I will dispute Ilfracombe given it's the other side of Exmoor to Barnstaple ( but I made my views on that clear before ), would *definitely* be good for Bideford/Torrington and it'd help Lyme a lot, but for the other Lyme Bay towns it's quicker just to bus to Exeter. Seaton is *supposed* to have a connecting bus to Axminster already, but the whole journey ends up slower than the X9. The WoE is somewhat unfortunately routed in East Devon, manages to be just out of reach of the coastal population.
Well, let me help. A new line, largely in tunnel, from somewhere on the outskirts of Exeter to somewhere on the outskirts of Plymouth, with no intermediate stations, would be in the region of £5-8bn. That’s based on the costs of ‘similar projects elsewhere’. Planning and development would be about half a billion.
Out of interest, if you have some idea what would be the rough cost of -
*) Wiring Bristol to Exeter
*) Wiring the B&H to Exeter
*) Wiring Exeter to Plymouth
*) Wiring and presumably resignalling Plymouth to Penzance. Would we be ready for ERTMS in time for that?
and if you have any idea, wiring Taunton-Exeter & Exeter-Newton Abbott ( does wiring along the sea *actually* cost more? ) just so we can estimate combinations. I'll attempt to avoid pretending to be a civil engineer but I'll assume new cutoffs on the current route would cost about the same per mile as the DAL plans.
I wonder if losing the view if a new line was tunnelled under the area would be an actual factor driving noticeable choices of mode - Plymouth isn't exactly a tourist destination but the area in general is. ( I also don't think you were really serious with that

).
Oh I’m all for improving links to Devon and Cornwall. Which is why after careful analysis, there is a massive building site just off shore at Dawlish. Be assured that a lot more of Devon will be affected before that new massive lump of concrete with tracks on top through Dawlish goes again.
Unless we want to abandon Dawlish itself, the protection from the sea needs to be ( and is being ) done regardless if there's a railway on it, yep. The Exe needs a new flood protection scheme all the way north of Exeter if it's going to be done properly too, although issues from the sea can't make it past Exeter at least. Just wait until the Somerset Levels start flooding more regularily...
If you want to re-open a railway from Tavistock to Plymouth the benefits (if not the costs) would be much higher if you re-opened the South Devon & Tavistock Railway to a new station at Plymouth Barbican (formerly Friary) via Yelverton & Dartmoor Parkway. Derriford could be served by a high quality cablecar with integrated ticketing from a new station at Plym Bridge which would also be the first in the UK to offer a hire canoe service. Of course the rail industry especially the stuck in the muds Network Rail would stifle innovation for such an obviously beneficial scheme.

( as an aside, someone genuinely attempted to run a sea-bus service along Lyme Bay, running the boat up on the beaches to board it ).
Where would an ideal commuter station in Plymouth be? nowhere a railway has ever been, presumably?