Here's my take, Psycho_Mantis:
Is it recommended to be playing on Officer difficulty?
Do what you want. There's no achievement/trophy tied to difficulty level. Cadet was easy, Officer normal, and I can't remember what hard is, probably Agito. I played on Officer and had to grind just once or twice. If you are getting 2 shot killed in Corsi Cave, you don't belong there yet.
Went to Corsi cave and the monsters were two shotting me. Things would be easier if I could have a healer. The cure command takes too long -____- . Who from class 0 is a healer?
Anyone can be a healer. Also don't forget to map an item to down on the dpad. I had a TON of megalixers by the end of the game and it felt weird using them. They're megalixers! You're supposed to save those, lol. Having a character KO'ed is part of the game, you will inevitably have to tag new members in to your team of 3.
This game is fun but there's so much going on. So many questions.
The arena....do the enemies ever change?
No. You don't really need this, it's good for some tasks that require "10 killsights" or whatever. I'd rather fight enemies on the world map if I need to level up
What does the yellow highlights on names mean before selecting a squad?
They're "primed." They'll perform better for that mission and will also have a red glow surrounding them in play, like Akuma from Street Fighter or some shit. There's an achievement for using only primed cadets for any one mission
The way the task system is handled is annoying. You can only choose one but many require you to go outside which takes 6 hours. I guess you can remember them, get the materials and come back but at that point why could they not just let you have multiple tasks?
Seriously. Also I hate how if you complete a task, you have to turn it in before accepting a new one. As you get deeper in the game the amount of time you have between missions will increase
Most important question: is it worth levelling Deuce up? Her buffs are pretty insane, do they stay that way?
It sounds like the common strategy is to pick your top 6 favorites and just focus on them. For my 1 playthrough on Officer (normal) I kept everyone at the same level. It did require like 2 nights worth of grinding during chapter 5 but I didn't mind it, combat is fun and I got better at it. Deuce was a great AI teammate for me but I rarely used her directly.