Question about fairness in a unique encounter.

So, in your scenario fairness is having characters that can NEVER miss? 

Nope that is a wrong interpretation.   Having a great skill bonus should not be negated 5% of the time, but that is not the to say they can't fail  either.  Rolling a 1 should not be the sole roll, there should be another roll or 2 (not necessarily a d20) that needs to fail as well.

Quote from Character77006 >>

rolled a Nat 20 which, in my game, negates the damage entirely

 

I feel this is problem number 1 which snowballs the other issues.  Rolling a 1 or a 20 is 10% of the potential rolls.  That should not mean an automatic anything.  Giving a 5% chance for a guaranteed hit/save is not always a decent choice like rolling a 1 should never be a guaranteed miss.  I had a character with a +21 and that 5% chance of a miss negates a great bonus.

DnD doesn't have many rules about special things happening on a 1 or a 20, except when it comes to attack rolls. For attack rolls, a 20 on the die always hits and a 1 always misses. I think that rule makes sense, because you should only be rolling dice if there's a meaningful chance of success or failure. You can have a +30 to your attack, but if you land a 1, it's still going to miss.

In the OP's scenario, I might have allowed the Rogue to use their Nat 20 to help the Druid, allowing advantage on the saving throw.

Anything outside of Ancient Dragons or Arch-Devils or Demon Lords will be an automatic hit, even if the character has disadvantage. To assume any warrior is so perfect that they can never miss or stumble in a fight is the definition of unfair. Now, if you play just for the mathematical precision of the constructed game, fair isn't a concept to be concerned with, I have a couple of players who would like to play that way sometimes. If you're more into the role play, no one is perfect.