Spoiler:
Show
1) The difficulty of each persuasion check is rebalanced to account for player level:
Difficulty = (Vanilla Difficulty)/2 + (Classic Level)/10
So for example the Feros Charm check (12 Charm points required) will have a difficulty of 8 at level 20, and 11 at level 50.
Very easy persuasion checks in late game are trivial in vanilla but made somewhat more difficult with this rebalancing. For example 4 Charm points required in vanilla, would mean a difficulty of 7 at level 50.
2) The availability of a Charm/Intimidate option depends on the difficulty and the number of Charm/Intimidate skill points:
Chance of a Persuasion option being available = Base% + 5% x (Skill points - Difficulty).
And there is a chance to get a critical success (option available no matter the difficulty/skill points), or a critical fail (option unavailable no matter the difficulty/skill points).
[Easy] Base% = 60%, critical success = 30%, critical fail = 10%.
[Normal] Base%. = 50%, critical success = 20%, critical fail = 20%.
[Hard] Base% = 40%, critical success = 10%, critical fail = 30%.
There are a few cases where a single conversation has mutliple Persuasion checks of increasing difficulty. In such cases there is only 1 random event, which will determine how far you can go (if you pass the hardest check, you automatically pass the easier ones as well). Otherwise, it would make these checks much more difficult because of the need to succeed 3-4 times in a row.
3) Examples
Example 1. The Feros Check is one of the hardest in the game (12 Charm or 10 Intimidate in vanilla). If we take it at level 30, with 8 points in Charm and 4 points in Intimidate, we have Charm difficulty at 9, so 45% chance to have this option. Intimidate difficulty is 8 so 30% chance to have this option. This means 61.5% chance to have at least one of these options available ("Normal" setting). With the "Easy" setting, it would be 55% and 40% -> 73%. With the "Hard" setting, it would be 35% and 20% -> 48%. So, even though we don't have enough skill points to unlock the dialog options in vanilla, there is a decent chance to have them with this mod.
Example 2. An easy persuasion check (4 Charm or 4 Intimidate in vanilla) but at level 50, with 12 points in Charm and 2 points in Intimidate, we have Charm difficulty at 7, so 75% chance to have this option. Intimidate difficulty is 7 so 25% chance to have this option. This means 81.3% chance to have at least one available ("Normal" setting). So it's very probable, but still not guaranteed. If we had invested the skills points differently: 7 in Charm and 7 in Intimidate, it would be 50% to have the Charm option, 50% to have the Intimidate option, 75% to have at least one.
.