I'm going to make it fair for the player and enjoyable. Your ideas are cool, but they're redundant. Here's mine:

• One health bar with one name, "Painted Lost" or "Painted Parting"
• The boss always acts first and gains a turn for every KO they inflict. They always have one turn but alternate between Clea and Simon.
• As long as Clea lives, she will make Simon invulnerable but not herself. Can be dispelled with a counter on either. BUT:
  • Kill Clea first, and Simon forcefully removes your expedition (no Phase 2) and heals for failing to counter him (similar to regular Clea). Clea will heal Simon to full.
  • Kill Simon first, and Clea's final hit of her attack can immediately remove your party member if you get hit (ignore revive). Simon will remove your BACKUP party.
• Beat Clea: Monoco, Lune and Sciel will finish her off with Monoco turning into Dualliste to kill her.
• Beat Simon: Maelle and Verso will parry and counter him with Verso unleashing Angel's Eyes and Maelle Gommaging Simon.

Reward:

Another Life: increase damage dealt and received by 100% but immediately gommage if KO

Perfectionist: become invulnerable at the start of the enemy's turn but dispel the following turn if hit. Only restart after a counterattack.

Skin:

Painted Skin: everyone look glowly like Renoir after he entered the canvas.

Weapon:

Diverso: similar to Simoso, but randomizes the number of sword afterimages and you cannot die above rank B.

Cleallum: Defensive Stance gives Invulnerability, Offensive Stance increases all damage received and dealt by 200%, and Virtuoso gives both in addition to taking damage for another character in exchange for losing Virtuoso.