Please check the text of Contingency: there is a minimum caster level. Contingency + Celerity is illegal for a 1st level caster.
You're missing how easy it is to boost caster level. Especially with the duration, there's no reason not to go balls-out at least once every so often.
Is the 1st level wizard casting 9th level spells required to be a Domain Wizard, or is there another way to gain the ability?
You can abuse taint and its super-Heighten with Extra Slot. All the other shennanigans I've got off the top of my head require Turn Undead or Bardic Music, so you need to be at least level 2.
A fun fact about the Taint trick, though: With a little bit of added work you don't actually even need the wizard level. I'm personally quite proud of making that little trick.
Versatile Spellcaster only gives the ability to cast spells already known. Am I misreading that, or do I have the wrong text? (Was the "you know" text added as a fix?)
If this text is correct, you could, as a 5'th level wizard, cast a 3'rd level spell (say, Fireball) using two second level slots. But you wouldn't gain the ability to cast any 4'th level spells with two 3'rd level slots, since you don't know any 4'th level spells.
The text is right but you're reading it wrong. All that's needed is the inherent ability to cast it and then Elven Wizard and Domain Wizard do their magic. It's like using Sanctum to qualify.
But, as I said, Heighten doesn't harm the build to throw in there.
Edit: That does bring up some questions: Could the same 5'th level wizard use two 2'nd level slots to cast a 1'st level spell which was metamagic'ed to require a 3'rd level slot? Could a 5'th level wizard use two 3'rd level slots to cast a 2'nd level spell which was metamagiced to require a 4'th level slot?
That depends on interpretation of metamagic and the quality of your helmet. A solid argument could be made, and this argument applies to Wish and Miracle, too, that since all it specifies is "level," you could use it for free metamagic. Just from the text, it's a reasonable interpretation, albeit one that will likely result in books being thrown at your head. And one that I personally despise, so I'd rather avoid going that road. Heighten is just special because it's Heighten.
I just want to add my two coppers to this one. In fact it is only one, notice the wording of Versatile Spellcaster "You can use two spell slots of the same level to cast a spell you know that is one level higher. For example, a sorcerer with this feat can expend two 2nd-level spell slots to cast any 3rd-level spell he knows."
Notice how this one gives the ability to join two slots cast a spell one level higher, but it says nothing about using two slots to prepare that same spell, a wizard with alacritous cognition can only use one slot per day for the purposes of spontaneous spellcasting, so he cannot benefit from it at all!!!.
Close, but no cigar. It does indeed say "cast," rather than "prepare," but you're forgetting that the specific trumps the general, and that Alacritous Cogitation and VS are entirely unconnected abilities.. In this case, the only result is that it allows a wizard to cast spontaneously by sacrificing slots