Oh god... you're going to drop Uncanny Forethought for Archmage? IMO, that sounds like a terrible trade.
Here is a build that I favor:
Spellthief 1/Wizard (Diviner) 5/Incantatrix 4/Unseen Seer 10 (in that order)
Race: Whisper Gnome, Human, other
Flaw: Vulnerable: -1 Armor Class penalty
Trait: Aggressive: +2 Initiative, -1 AC
Feats: Able Learner (flaw), Mercantile Background (Human), Extend Spell (1), Scribe Scroll (wiz1), Item Familiar (3), Spell Mastery (Wiz5), Uncanny Forethought (6), Iron Will (Otyugh Hole), Persistent Spell (Inx1), Alertness (Item Familiar), Master Spellthief (9), Quicken (Inx4), Craven/Insightful Divination/?
Advanced Learning: Hunter's Eye, ?
So this guy can do all the stuff you care about as an Incantatrix, starting as early as levels 8-9, plus he can also perform amazingly well as the party sneak/rogue, and dish out insane damage via his Sneak Attack (especially kicks in at levels 12+, but he's still nasty at the earlier levels). Master Spellthief will let you trade Persists for buffs with your party cleric (and others), meaning you can run around with Persistent Divine Power or whatever other insanity that you want. Make sure to go crazy loading out your spellbook, as you can spontaneously cast anything from it via Uncanny Forethought. Even at 7th level, my guy's spellbook looks like this:
0th: KNOWS ALL and all these are in his spellbook
Detect Magic (x2, 1 in specialist slot)
No Light(BoVD): 20' radius area of magical darkness that creatures with darkvision can see through.
Mage Hand
Dancing Lights
Prestidigitation
Launch Item: "Throw" fine-sized item up to medium range with no range penalty.
Caltrops (SC): Creates caltrops in 5 squares within short range, and they get a +4 to hit (ignores armor, shield, deflection, +2 AC if wearing shoes or armor).
Ghost Sound
Message
Acid Splash
Open/Close
Ray of Frost
Resistance (+1 saves for 1 min)
Touch of Fatigue
1st:
Alarm
Benign Transposition(SC): Two allies within medium range trade places
Comprehend Languages*
Eyes of the Avoral
Feather Fall
Grease
Instant Diversion x2 (RotD, swift action, creates 1 illusory double +1/4lvls (max 5), 1rnd dur)
Lesser Orb of Acid(SC): 5d8 acid damage, short range.
Mage Armor
Magic Missile
Chill Touch
Ray of Enfeeblement
Nerveskitter(SC): Immediate action +5 to initiative
Obscuring Mist
Protection from Evil
Silent Image
Unseen Servant
Wall of Smoke
Uncanny Forethought (x2)
2nd:
Alter Self
Cloud of Bewilderment (basically mini-Stinking Cloud, SC)
Detect Thoughts* (specialist bonus)
False Life
Cloud of Knives
Glitterdust
Gust of Wind
Heroics (SC, 10 min/lvl, gain [Fighter] feat)
Invisibility
Kelgore's Grave Mist(PHB2): medium range, 20' radius, 20' high cloud does 1d6 cold/rnd and fatigues. No save, SR to resist fatigue. Lasts 1 rnd/lvl.
Knock
Chain of Eyes (SpC put this on the Wiz list I think)
Resist Energy
Rope Trick
See Invisible
Spider Climb
Toothed Tentacle
Web
Uncanny Forethought (x2*)
3rd:
Arcane Sight*
Anticipate Teleport(SC): 24 hr duration, range touch, anyone teleporting into a 5'/level radius is delayed 1 round, and target knows they're coming
Blacklight
Dispel Magic
Gaseous Form
Ghost Lantern (CMage, 10 min/lvl, creates a torch that only the spell's targets can see!!)
Shatterfloor (SC): medium range, 15' radius, 1d4 sonic/lvl and area becomes difficult terrain
Clairvoyance/Clairaudience
Heart of Water (Swim speed = land speed, Water Breathing, +5 Escape Artist, can discharge as Swift action for FoM for 1rnd/lvl)
Nondetection
Phantom Steed
Shrink Item
Uncanny Forethought (x2)
6th:
Contingency
Spellbook cost:
1) Instead of buying scrolls, he paid another mage to let him copy his spellbook for all spells above 1st level. This only costs 50 gp per spell level, which is awesome!
1st: 18 total - 9 free = 9 at 25 gp (buy scrolls, 225 gp)
2nd: 17 total - 4 free = 11 at 100 gp (pay spellcaster 1100 gp)
3rd: 13 -4 free = 9 at 150 gp (pay spellcaster 1350)
6th: 1 (pay spellcaster 300 gp)
His spellbook currently only has 43 of its 100 pages filled. The rest of his spells are stored in an Aurora's Shard, which is like half of a Boccob's Blessed Book (can scribe spells into it for free). The shard has 53 of its 500 "pages" filled.
I'm using an owl as his familiar, as I plan to operate mostly at night, which gives me a +3 to Spot and the owl is just insanely good as a scout on this build since it gets to use my ranks in Spot and Hide (which I actually have).