MJ
In 300 hrs with a GPS (KLN94B) I've never seen a RAIM warning. If yours doesn't have a rooftop aerial, it will have marginal performance at best. What model is it and how old is it? If your aerial is on the roof, is it an active one or a passive one? Do you always load the QNH into the GPS at startup?
FS
I agree with #4, in the PPL context.
For IFR, GPS/VOR/DME/ADF concurrently, whatever is available, plus the plog and the chart. The GPS course on the map display is the primary reference and the others must confirm it. Shame PPLs aren't taught to fly that way too.
boomerangben
You can always find these stories from somebody. Was it an ancient GPS with an internal aerial, used below deck perhaps?? Or did they enter a user waypoint and got a couple of digits transposed?