How can I force the 955 to use DEM instead of the onboard sensor for he altitude data?

Unfortunately, no. You can "turn off" the altitude reading, but then your outdoor workouts will always say 0 m elevation.Since my barometer I broke, I manually correct the elevation of every single workout in the Connect web version

I like you also have a faulty altimeter/barometer sensor. It's a bit of a pain to manually correct the altitude each time. That's what I do at the moment. I can live with having it as 0 elevation. 

How do you turn off the altitude reading? 
I can't seem to find that option. 

Its a shame that it broke after 3 years already. The watch is probably lasting another 5 years or so, so I have to just cope with it. Fortunately, I live in a very flat region and I do trail runs with elevation only rarely.

To turn off the elevation readings, you head into the elevation glance -> settings -> sensor mode -> only barometer. What this does is that it tells the watch that every change in air pressure is related to a change in weather, but not of elevation