PPruneRadar
Your posts begs the question with your familiarity with recent GPS moving maps.
The only times I can remember inputting a lat long co-ordinate is when going to a grass strip not in the database. Yes there is plenty of potential for error and you would want to check before you set off that the position on the moving map is roughly where you expected the strip to be.
However to your example of an unplanned diversion round CAS. It is simple. Personally, I would follow the boundary until reunited with my original track. Not too demanding and whilst you make the point of the need to ensure you follow the correct boundary on a colour moving map the boundaries are in my opinion far better defined that on a chart.
Alternatively, and this is where I wonder about your familiarity, move the cursor to a suitable spot outside CAS (any where you like really) it takes a few seconds, hit GOTO and there is the new course. When you reach that point pull up the flight plan and hit GOTO again for the next point you would have gone to outside CAS on the original “flight plan” and there it is.
Finally I don’t recall anyone saying GPS was infallible, what I do recall are pilots observation that the reliability of GPS compared with radio navigation aids is very favourable.
So what do you do if it fails. If you can, use the radio navigation aids in the cockpit, use dead reckoning from your last know position and / or use visual navigation. Depending on the individual the skills might be a bit rusty BUT if they are get a steer or two from D and D. They will oblige, it is not a drama, and if the failure rate is once in 10,000 hours I don’t suppose they will be too over worked.