Wizards - some build questions

[DrFunktion]

Not looking to start an argument on whether wizards are balanced / OP, just a few points of advice / clarification:

1. Am I right in saying spell use per encounter is only ever 1 for your highest level of spells (empower aside)?
2. If so even a level 20 character would only have 1 cast per encounter of a level 9 spell (empower aside), right? However would taking the prestige passive increase this to 2?
3. Back in the BG2 days I used to love spell triggers and contingencies and unfortunately (for me) there doesn't seem to be anything similar here. Would I be right in saying that there is no mechanism for emulating this, through reduced recovery time for example?
4. On similar lines, rapid casting looks appealing but 10% doesn't seem that much. Any advice on whether or not this is worth taking?
5. More generally is a 'kill them before they kill you' approach (just focusing on cc and damage) viable or are some defensive abilities a must? I imagine this depends on difficulty and whether or not you are trying to solo, so for sake of argument let's say I was trying to solo on classic difficulty.

Thanks in advance!