In short, answer to your questions would be NO, NO, YES!

In long:

No! You don't need special knowledge for replacing print head.
Basically, to replace print head you have to (after taking of the covers) disconnect cable and tube and unscrew two screws!
Only tricky thing that is alignment of the head (when it is replaced).
For that service technicians have special tool. Of course that you can do it manually without any tool - it is time and nerve consuming process, but it is feasible!
For example, easiest way would be to mark position of old head and replace it with new one over those marks!
OK, I wrote this in short just to assure you that it's not so hard!

No! You don't have to buy it from HP! You can buy it from any company that sells printer spare parts!
HP DesignJet 9000s and 10000s aka Oce CS6060 and CS 6100 aka Seiko Colorpainter 64s and 100s uses Konica Minolta 512 MN print head.
Price of 3200$ is really to much.
For example, I'm buying everything from one company placed in Brussels! They offer those heads that you need for price of 1190 Euros which converted to $ is exactly 1600.
Check this link!
Ofcourse that you can probably find lots of companies that sell spare parts that you need closer to your location. Just google for it!

Yes! Many of us here have service manual and are willing to give to strangers like you....

awesome thanks for the help.

as long as it is not user caused? when it is not user caused?

all errors can be described as user mistake!
if you exclude obvious mistake of head hitting surface, dried ink and clogged nozzles are users error of not doing good maintenance of his machine.

if you buy those cleaning solutions (like cleaning sheets which has price of 115 and much more) than you can say and prove to them that you did everything possible (from simple user point of view)! but what would be cost of that?

when you say that whole machine is under warranty, does it mean that if, for example, Main PCA board (IPB1) dies you don't pay anything, or it means that you have privilege to buy it for "special price" of what it actually cost?

Never had extend warranty for any of machines that I work with, so I have no experience in deals (contracts) like that!!