You have to break a project like this down to some general categories of constituent part and study those aspects independently rather than trying to accomplish a bunch of things that are new and unfamiliar to you in one project. That will just waste a lot of time; in the beginning people tend to think the wrong way around, that just getting on with their ultimate goal is the fastest way to proceed.
By analogy, if we decide to go to LA from New York and you say, "Right, let's get on with it!", slip on some sandles, grab some change, and walk out the door thinking you will get there faster than if you learn to drive, save some money, order a ticket of some kind, etc., you are probably wrong and even if you do make in a decent amount of time -- say, hitchhiking -- you will eventually realise hitchhiking is not a reliable way to get around large parts of North America and anyone who does it regularly has invested in some alternate strategies.
The thing which jumps out here is GUI programming. Choose a language, choose a widget toolkit (e.g., Gtk+), and go through some tutorials.
Your Pi specific questions are really a hodgepodge of tangentially related things, if you need more details you should drill down on them individually.
Can I make a Pi2 talk over a network?
Yes, although if you mean wireless network you would need a wifi adapter for it.
I presume you already know this, and what you really meant is how. This the realm of network programming, and if you don't know anything about it, you will have to put in some time.
can utilise my PI2 as a HID
Not directly in the way that the Zero can.
is it possible to do this using a USB to TTL Serial Cable?
Sure, although again I suspect what you meant is how. The short answer to which is connect each pi's TX to the other's RX (actually, that doesn't involve a USB to TTL cable, it's just two wires), and use whatever UART software you want --
miniterm.py, etc. You will have to write your own code to do what you want; the advantage of this is that it is considerably simpler than IP networking.