no problemo, to adapt a known saying... there are no irrelevant questions, only irrelevant answers :-)
a) Module interconnections/"power line"
I´d recommend IDC connectors, as available in SmashTVs shop: http://www.midibox-shop.com/buy.html
They are available with different numbers of pins for all your needs.
of course, you also need their counterparts on the veroboards, they look like this: http://www.reichelt....00&PROVID=2402
Regarding cabling/splitting the wires... for the IDC connector, you just insert a ribbon cable and press it together... a small tutorial can be found here:
Using these ribbon cables, you can also avoid "Y-Junctions", by just pressing on a connector in the middle of the cable and pressing on another one a bit away...
If you only have 2 pins for the power line, this is a bit overkill, but you could create a +14V, +5V, GROUND, GROUND, GROUND, GROUND power line using a 6-PIN IDC connector.
The good thing using these connectors is, that they are "coded", so a wrong connection is impossible. And you can separate your modules afterwards (reduces cable spaghetti a little bit :-))
b) where in the "power line"
it should not matter where you plug that mini-keyboard-"psu" veroboard... before or after the sid modules...
c) 100mA
...to my knowing are enough for a SID-.. That´s why a low-power 78L09 (or ..12 for the 6581 sid) in the mentioned schematics is enough to drive those. As you need 500mA for the keyboard, i´d recommend using two normal 7809s... one on the "optimized PSU" circuit side, and one on your "mini keyboard-psu" veroboard.
d) cutting the schematics
you need to start at C10 (330uf) and work your way rightwards (using a 7809 ofcourse)... then you will have a clean separate 9V out for the keyboard.
Please test appropriate voltages on all modules (SID, Core, Keyboard-Mini-PSU) before plugging in "active/expensive" components, like the keyboard, or any ICs (SIDs/PICs...)
Have fun!
Peter