1)Replacing the headband i feel it will hurt, my DC are important to me, as the interactions i could get roleplaying (I'm the party face).
2)I could ask to my cleric to cast some spell that gives chance of missing on me...will that solve the problem?
3)Shield doesn't convince me because of the duration of the spell; Magic Armour has a great duration and it gives +4 AC too.
Shouldn't i get a wand of that spell instead of burning a spell known slot?
4)Is Web a weak spell for my level? In that case i could swap it for Mirror Image.
Invisibility with Glitterdust could be a nice shot, because anyway the summoner already have Invisibility, and i think he could cast that on me.
5a)Does Pages of Spell Knowledge let me learn a spell from another source than mine (arcane) or "just" (and i'm not meaning it's a poor option) arcane ones?
5b)Does Mnemonic Vestment works with Pages of Spell K.? If i have understood this item doesn't consume the scroll or so, using it, am i wrong?
Answers in order! ;)
1- Replacing the headband with a lower level version does hurt some things obviously, but with such little gold to spend at your level (you're basically a level behind Wealth By Level recommendations), it's just a matter of prioritizing.
If you're happy spending the 36k gold for the full +6, then go for it. But downgrading to a +4 saves you 16k gold you can spend elsewhere. In the end, you just gotta answer this one for yourself. ;)
2- In place of a Mirror Image? No. Clerics have some great defensive spell options, but Mirror Image is one the best personal physical defense spell in the game (others being things like Greater Invisibility or Prismatic Sphere, basically Arcane spells that the Cleric likely won't have access to).
The Cleric absolutely can cover things like Communal Resist Energy though.
3- You're totally right on Shield. :) Learn Mage Armor and keep a wand of Shield.
4- Web is a fantastic spell at lower levels, but as you gain levels, you gain better options, like Create Pit and Black Tentacles.
It's worth it to swap out Web for something else, IMO.
5a- No, you can only learn spells that are on your list (Sorcerer/Wizard arcane spells for you, Cleric/Oracle spells for an Oracle, Bard spells for a Bard, etc.) with Pages.
5b- The Mnemonic Vestments work with any written spell (scrolls, spellbooks, etc) that is on your spell list (Sorcerer/Wizard) and of the same level or lower as the slot you use to cast it.
I'm sure technically you could use it to cast from a Page of Spell Knowledge, but that would just be a waste of the Vestment's usage, because the Page of Spell Knowledge just adds that spell to your list of Spells Known. :)