1 level in mana recharge is sufficient. I'm maxing meteor and drain, but i heard trap is also useful since lots of later mobs use brutal/seths.
Also, consider waiting on speccing scud, new patch (evo) will eventually bring a new staff aoe main only skill, which does tons of damage and is > than scud in nearly every way.
max blessing/meteor (the new staff aoe) is ideal.
I can say that if you don't have scud maxed, probably you'd want a different merc, maybe another spear or an art instead for stun/physical pressure/ap drain. scud is good when stacked with beast summon / meteor. Not to say you can't get a witch, though.
Exo you'll want to max seal and (possibly) speed cast. Manaburn's primary purpose is in conjunction with mana trap/drain to stop a seths will line in pvp (no mana mercs w/ seth = can't attack).
Oracle will probably be replacing your shaman, in your theoretical build I'd probably replace your witch.
From what I've heard about mana shield, it uses your mana in place of your hp when you take damage. I heard it does an adequate job of reducing damage, until you get focused, in which case your mana pool goes baibai. Debatable; may be worth getting a few points, it remains to be seen.
Your proposed formation is a little bit of everything, you might want to hash out a strategy in order to get a better feel of what to tweak/change; are you going for stuns/aoe/freezing middle line after focusing front/etc. etc.
if the post above this is insightful/useful, rep me plz