Nope. 1/20 is much less regular than 5/20 or even 19/20.
What do you mean here? Any roll is as likely as any other
Do you mean 2-20 is more likely than rolling a 1? Of course it is, but an invisible rogue sneaking at +15 shouldn’t be seen by the monster who’s -4 to spot 1 in 20 events, or if 20s are also special, 1 in 10 events (one for the rogue getting a 1, one for monster getting a 20)
What do you mean here? Any roll is as likely as any other
Do you mean 2-20 is more likely than rolling a 1? Of course it is, but an invisible rogue sneaking at +15 shouldn’t be seen by the monster who’s -4 to spot 1 in 20 events, or if 20s are also special, 1 in 10 events (one for the rogue getting a 1, one for monster getting a 20)
In that case, and I keep repeating myself: don’t roll.
Don’t roll for things that can’t fail.