The route is along the river estuaries, fairly sheltered and not exposed to direct ocean storms and tides. Nothing like the conditions at Dawlish, though like everywhere low-lying, rising sea levels may become a major concern in the very long term.
Instow crossing would have to be reopened unless the road concerned could be closed, as building a bridge of any sort in the vicinity would be eyewateringly expensive and horribly obtrusive visually in such a picturesque waterside location. ORR has softened its stance on the 'no new crossings' edict, allowing three new ones on a heritage operation in the south of England, for example.
The recently introduced Automatic Full Barrier Crossing Locally monitored design might offer a solution for Instow. This has full barriers closing the road entirely, obstacle detector technology to prove the road surface within the gates is clear and is locally monitored by the train driver observing the crossing visually and the flashing crossing indicator before proceeding. The driver's indicator changes from red to white once the control equipment has gone through its expected sequence correctly, the barriers are proven down and intact, and the red flashing road signals are illuminated. With up trains starting from rest at the station, and down ones approaching slowly, expecting to stop at the platform, the necessarily low crossing speed defined by the approach visibility should not be a problem operationally on such a line. The crossing area must be illuminated automatically on a train's approach to ensure the driver can observe the road at all times of day. The obstacle detection, local monitoring and full barriers should make such an installation just as safe if not more so for pedestrians, cyclists, and motorists than a typical light rail intersection with standard traffic lights alone. Another benefit of the AFBCL type is a short road closure time for each train movement, more like the ~30 seconds Automatic (half) Barrier Crossing Locally monitored (ABCL) warning than a typical Manually Controlled Barrier with Obstacle Detection (MCB-OD, actually fully automatic in normal operation!), that can sometimes be closed 2 or 3 minutes before a train arrives.