Draw a scroll (lets say from a Haversack), and then cast the spell defensively?
And if so which stat do you use? Or can you choose your highest mental stat? Or do you always use your Charisma since you're using the UMD skill?
First you would need to use UMD to 'decipher a written spell' or use read magic. Just because someone tells you what is doesn't mean you can read it.
Then you need the appropriate ability score for the class that scribed the scroll, or another UMD check to ‘emulate an ability score’.
Next you need another UMD check to ‘use a scroll’.
Finally, using a scroll is a standard action that provokes attacks of opportunity exactly as casting a spell does. So, I guess you would need a concentration check of (making this up now) d20+character level?+ability score you emulated bonus?
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No.
Spells are cast from your mind. Scrolls are read from paper. Let me quote:
To cast a spell, you must concentrate.
Casting Defensively: If you want to cast a spell without provoking any attacks of opportunity, you must make a concentration check (...) to succeed. You lose the spell if you fail.
Activation: To activate a scroll, a spellcaster must read the spell written on it. This involves several steps and conditions.
Since reading a scroll doesn't involve concentration (you're just reading), you cannot make concentration checks, and you cannot cast defensively to avoid losing concentration.
Also note that scrolls do not waste the spell if your reading is interrupted:
A spell on a scroll can be used only once. The writing vanishes from the scroll when the spell is activated.
If you want to house-rule reading scrolls defensively, that'll be fine; but I don't think RAW allows for that.