>main problem is limited parking at Okehampton and Exeter Station is on the outskirts of the city centre
When Okehampton had its original station, trains used to run to Central. I haven't read anything about Central being closed so I'm presuming it's still there?
>This line originally connected Exeter to North Devon and alternate route to Penzance Cornwall.
Only an "alternate route" as far as Plymouth. There were branches to Cornish north coast towns like Padstow and Wadebridge but Penzance was only ever GWR.