Whoa whoa do NOT use 17v, it doesn't work that way. And you'll fry everything.
wow, ok lets try to get this covered.
Your screen uses 12v, and the raspberry pi uses 5v, you do NOT add those together.
If your main powersupply is 12v, you can connect the screen to it directly.
But you will need to use a 5v power regulator to power the pi. So the power regulator would step the 12v down to 5v. Using a high quality 2amp usb charger for a car's lighter plug would work fine. since cars use 12v, and it'll step it down to 5v for USB. No generic $5 chargers, must be name brand, preferably one made for tablets like the iPad.
Buttons, if connected to the GPIO do not need external power. Joysticks (button based) do not need external power. Analog joysticks such as thumb sticks from an xbox/ps2 controller, need an analog to digital converter and a bit of programming, these can be powered by the GPIO pins since they're very low current.
Speakers will likely need external power and a power amplifier. The audio power from the Pi is very low, which is fine for headphones, but for actual speakers, it won't be enough, you'll need to use an audio amplifier, and those require external power.
Ok now that's out of the way, voltage and ampers are not the same thing.
if your power supply is 12v, it'll give 12v to everything connected to it. One 12v supply can power two 12v devices.
The thing to note is the ampers, the power consumption, the current needed.
If your power supply is 12v(volt) 2A(amps), and you use two 12v devices that both draw 500mA (0.5A) it'll be fine, since 2x 0.5A = 1Amp, which is half of what your power supply can offer.
If you use 2 devices that each consume 1.5A, it won't work, since 2x 1.5A is 3A, which is more than your power supply can put out.
You're going to need some help here, so before you get started with wiring and testing stuff, it may be a good idea to get some books on how electrical components and such work. Maybe take a class, since what you seem to want to do, isn't really for beginners. Maybe someone can suggest some books for you to read to help you learn this stuff. It's not easy, but is rewarding to know.
My RPi needs a fan, heat will leech into the battery, so people saying I don't need a fan, don't understand how Li-Po batteries are affected by high temps. Cool pi = cool battery.
I would very much so like to see a Pi2/Pi3 Zero, power and size.