Oh, my, if it wasn't right before lunch, I would answer all those questions for you.  Since I am about to eat, I'm sorry, but I'll only answer these few.

There is no complete documentation for the calculator.  If you know z80 (And I mean really well) you can look at brandonw's svn (which I will not link to, nor should anyone link to anything of brandonw's.  PM me for a link) and learn from that code like I did/am doing.  This page isn't complete, but has quite a few resources with it.  Definitely be sure to go down to the very bottom under the "In other News" section.

EDIT: Also note, that even brandonw and I can't give you complete documentation.  I usually go to brandonw for questions, but I would definitely study his code. (again, I'm not putting a link here)

Basically you just find a female a and a male mini-b or a (either will work atm, preferably mini-a b/c there's less work for later (and yes I know in the end it has to be mini-a but I dealt with that ??)), cut the cables, strip off some of the outside shell.  Keep the mesh-like stuff on.
Strip the colored wires on both sites, twist like colors together and wrap with electrical tape.
Kind of twist together or whatever the meshy part, tape together.  Wrap whole assembly in electrical tape.

Now, look at the smaller plug.

Which one does it look like?
If it's A, you're done and ready to use it.
If it's B, read this
http://ourl.ca/7646/142328
or get yourself familiar with a soldering gun

You'll need to link pins 4 and x somehow to make it work.
http://pinouts.ru/connectors/usb_a_b_male.gif

As for the omni link, it's supposedly a "temporary" way to do it but I've had it that way with mine for quite a while now and it works..

[wikiti]msd8x[/wikiti] [wikiti]usb8x[/wikiti]
Useful info and stuff.  Good idea to browse through for a while.

EDIT: there's also [wikiti]USBtools[/wikiti], which allows you to do stuff like use a keyboard or mouse.

If you have any more questions, feel free to ask

For the record, I wouldn't strip the USB cable.  I just bought a mini-A to A female.  It was pretty cheep, IIRC.

++

I use something like this most of the time.  Just not many people have access to that sort of thing
For the record, I'm using a converted adapter from this kit and it's great, tho there are definitely cheaper alternatives out there.
imo it's a good thing to have a kit like this anyway =P
http://www.walmart.com/ip/Emerge-Tech-3-USB-2.0-Universal-Cable-Kit/12575355

Headphones are normally 3.5mm.  The calc's port is 2.5mm.
(iirc?)

Thanks. What music creation utilities are there for on-calc music editing? If there's none, I'm totally making one.