Looking at the 1965 timetable on
www.timetableworld.com, it appears that the fastest westbound service from Exeter to Plymouth was 1hr40, the fastest eastbound is 1hr30, stopping in each direction only at Devonport, Bere Alston, Tavistock, Okehampton.
Much as it would be great to see this line reopen, I think the reality of the situation is that any replacement to the sea wall line is going to be south of Dartmoor. That's where all the population is (both Okehampton and Tavistock are sizeable, but there's nowhere else of note between Crediton and Plymouth), and it avoids reversal.
The diversionary route with two reversals would take, say, 1hr40 from arrival at Exeter to departure from Plymouth, allowing for ten to twenty minutes minutes speeding up since 1965. That would be a journey time of 3hr40 from Exeter to Penzance. You can drive it in just over 90 minutes.
I can't help feeling that the South West beyond Exeter is somewhere where a regular (i.e. hourly) railway run express coach service from St. Davids forecourt might be the best solution, not as a replacement to the railway, but as an addition, which would ease the problem when the railway line got cut. If you ran the express coach service Mon-Fri, you'd also then own a fleet of coaches and railway employed drivers to move to whichever part of the line is closed every weekend.
I know it wouldn't be an entire solution, but it would be a quick relatively low-cost start. I'm sure I'm not the only one who has given up using the railway line down to the south west following a number of horrible travelling experiences, which weren't so much the fault of FGW, but down to the complete lack of alternatives when things go wrong.