5k Marks is enough for two main pieces of gear. I would be better off making a new character, leveling monk to 50, and gearing my Monk and WHM separately for PVP.
No, you wouldn't, because you wouldn't earn seals any faster by playing alts than you would by playing on one character. In fact, playing alts would slow you down, because your rank on each alt would end up rising slower than if you played different jobs on the same character.

For this reason, PvP does not really discourage armory system use. The only real issue is with PvP actions. You'll only get a maximum of 30 AP, so that means 10 actions learned or fewer upgraded ones (though most of the upgrades aren't too worth it). Some of those actions can be used by many classes (purify is available to all) though, so the impact isn't that major.

The real reason for the "normalize pvp gear" thread comes to light here. You're just qqing that you can't immediately get rewards for every single job at the exact same time.