Trouble is, as FGW can *currently* keep a semblance of some service running, the Okehampton route is only seen as a worst case scenario potential back up.
With Level 1 working such as Tuesday gone, no trains would have been diverted via Okehampton; XC would still have been curtailed at EXD. Therefore with an operational FGW timetable, why would anyone need to go via Okehampton when it would only be a 15 min delay via DWL?
Even with Level 2 working, it's very unlikely that anything would be diverted too. The only time diversions would happen is if DWL was impassable again for a long period of time or any stock cascaded after IEP have the same problems as voyagers.
This is why Okehampton is not seen as viable by the government - all that faff for a "handful" of days a month, with little [predicted] return as a 'normal' railway.