For reasons I'm unclear on, Reis's Ice Breath obliterated those marlboros
The reason is that all three tiers of Malboro are weak to ice. Many monsters have elemental weaknesses.
a Mystic that can hit hard.
Normal Mystics can hit hard, too; Poles have range 2, like spears do, the best pole is WP 16, and Poles damage is MA x WP. As long as you tweak equipment correctly, it's perfectly possible for a Mystic physical attack to hit as hard as any physical class, you just need to max MA.
how the game could manage to have all twelve Zodiac Stones result in twelve Lucavi boss fights
I mean, theoretically speaking, Ramza is currently carrying five stones, right? Aries, Taurus, Scorpio, Aquarius, Pisces. It's not impossible that a group of three enemies could steal three of those at the start of a battle (let's say Taurus, Aquarius and Pisces), turn into Lucavi on the spot, and you have to face three of them at once. That'd be an interesting battle, right?
Just saying.
Likely--If I remember correctly, the game only gives you just enough slots at base to keep all the characters who ask to join you, no more.
In WotL, yes. In the original, it doesn't - you have sixteen slots total, which is not enough to cover all the characters you can get. It's one of the few changes WotL made that I approve of.
The final leg of the quiz is a test of 'find a prime number' that infamously doesn't consider 1 a prime number.
As others mentioned, certain formulations of how to calculate a prime number stick to that; Final Fantasy Tactics itself does, when it eventually brings Prime Numbers into the game, so obviously the FFXIV references to it would stick to that formulation.