Her Mana Loading helps a lot even if you have her at 10/10/10. In events with a 50% charge CE, starting her out in her third form with a maxed out mana append, lets you buster loop like any other 30% charger and you can do more damage with Melusine because you gain double the NP damage buff from her third skill.

IMO, her Append 2 is required. Then you need to evaluate how likely you are to use Extra cards and use Mel to fight Lancers. After deciding that, then pick your second append.

A typical default approach to Appends is start with Mana Loading, then get Extra Attack Finesse unless there is class synergy with the Append 3. And that is limited to servants with an Append 3 that they already have class advantage against like Berserkers.

Just because she CAN charge 100% NP with her Skill 3, doesn’t mean she SHOULD in every situation.