Regarding the difficulty for the average golfer vs the scratch golfer, perhaps someone with an expertise in how courses are rated for handicap purposes could chime in. Aren't the #1 handicap holes typically the longer ones despite their difficulty, simply because it gives the scratch golfer more of an opportunity to separate themselves from the average player (because the par is higher and therefore will consistently require more shots)?
I am no expert but I am pretty sure holes are handicapped by actual scores. I know the course where I play most frequently re-handicapped their holes several years ago. They collected score cards for the entire summer. The #1 handicap hole was determined by figuring out which hole had the greatest differential between the "scratch" golfer and the "18-handicap" golfer. The spread was in actual strokes, not percentages. Almost by definition, par 5s will tend to have the higher stroke differentials.
Couple this with the fact that all front 9s have odd handicaps and back 9s have even handicaps and it all gets pretty watered down.