Those two resistors, marked RN4A and RN4B, are used to protect the uart line from conflicts, while also assigning priority to the external header. They act like weak pullups, in that anything before (In this case, left of) the resistors will only affect the line after them, if the lines are not being used.
Anything serial connection plugged into the header, will take priority over the Arduino's usb/serial connection, as they can directly power the lines.
But this will also provide issues during programming, so use an enable pin, or power switch, or simply remove the bluetooth module while programming.
First, I'd say dump the wireless requirement, at least for the early prototypes. Once you have a prototype that works, and you've picked up some electrical knowledge along the way, you can add-in wireless after-the-fact. Assuming you've designed the firmware well, it should be fairly easiy.
Then, I would say the approach I would recommend would be to target a microcontroller that can easily emulate a HID device.
The cheap and easy approach, and the one I would take, is to buy an arduino leonardo. The leonardo (and the makey makey, for that matter), both use an ATmega32U4, which is a microcontroller with an integrated USB interface.
Since the USB interface is part of the microcontroller, rather then a separate, purpose-specific device, it can be configured to act as a arbitrary HID (human interface device). In fact, there already exists a library for using a ATmega32U4 as a USB keyboard.
Now, lastly, you are basically almost certainly going to have to use a switch-matrix of some sort. Aside from designing your own circuit-board, with an enormous IC (such as a 144 pin TQFP, or similar), you are not going to have enough IO lines to have a dedicated input for every key.
This is fine. Switch matrices are a well-understood practice, and if you're really concerned about button aliasing, you can add a diode for every switch, and make the circuit-board incapable of aliasing.
For the moment, I would suggest you buy an arduino leonardo, and throw together a prototype. I think you're underestimating the mechanical complexity of this build significantly, and having the electronics you need to at least get the system talking to the computer, and acting as a keyboard will let you start poking around at the mechanics.
Best Answer
You need something called a datasheet. That will have all (or most) of the information necessary for a knowledgeable designer to use the parts.
In this case, you know what the part is: a switch
You know the model series: MX
You should know the manufacturer: Cherry Americas LLC (owned by a German conglomerate and probably manufacturing the switches in Zuhai China, near Macau).
So google that and you should come up with this datasheet. A similar one is here Unfortunately I don't see any with an odd number of contact pins, unless you count the plastic locating pin in the center.
Anyway, all the information you need should be there for whatever type of switch you are looking to purchase. Unless you want the kind with a matrix diode or LED incorporated, in which case contact the manufacturer to find out the polarity of the diode(s).