@Post #5 With programming experience, one can say this suggestion is no harder than whats required to make a Monster's stats/damage/Hp/Mp scaled to level.
A Hero's stats and resistance is dynamic, which is a programming term that means it values change immediately following an action to modify it like changing equipment, static is a term that means its values are set to a certain value initially and never changes like a Guest's max Hp and Mp that is based on the Hero's at the moment of inviting them to the party in the current game engine.
Modeled after the Hero, its only logical to have a Guest's stat values be dynamic and not static to respond to changes executed in combat and make stat debuffs functional as statuses in the foreseeable future.
Logic goes that if certain class can reduce an enemies' Immobility resistance why can't an enemy do that to address the Hero's/Guest's Immobility resistance.
In summary, Like Hero, Like Guest, Like Monster.
@Post #6 I think I explained very clearly how resistance changes the 1x Hp of the Hero's relative to 1x Guest's in the first paragraph directed at Post#3, the second paragraph you quoted is based on that.
I think you either missed or have ignored the entirety of the example of the ratio in bonus/damage debuffs. 1x is a debuff at maximum potency at a one turn duration, obviously potency and duration are directly affected by each other and power can be distributed at any value between them.
With that said, I have no idea why you think standardizing a ratio to prevent invincibility when Guests are at play would produce sameness, it does not produce that effect whatsoever.
Standards are an integral part of balance as its creates a foundation where all variability produced from it would be tied to mathematical sense, this is the only guaranteed method to keep variability in check as subjective attempts to gauge for balance in an RNG involved performance will not produce a 100% consistency.
Since you are thoroughly convinced that my suggestion is a pure buff as indicated in all your posts, its extremely likely that you don't understand how MIN works, in short its an overall nerf to Cha.
Currently a Hero can train Cha and none in Str/Dex/Int, the result is a Guest is stronger than the Hero and once again the +60% Enemy damage per Guest fails to represents it as the true value would be beyond +100% if properly represented.
MIN means that it takes the smaller of the two numbers in the (), that means Cha alone will not affect Guest damage if the Hero doesn't have Str/Dex/Int, it becomes mandatory for the Hero to train at least one damage stat and use a Guest whose attack type matches it for stat damage to become relevant.
If one makes the poor decision of a Guest's Str/Dex/Int scaling to Cha directly, that would mean it increases in a manner that is at a triple growth relative to the Hero training in one stat, namely in non-critical damage, damage bonus with DoT/HoT applied and bonus to critical damage multiplier.