Developing Standalone Arduino GUI Controller I am rather new to using an Arduino. 
As a part of my Graduate-Level Chemistry research, I am trying to further the development of a cheap DIY, open-source, Flow Chemistry Syringe pump system.

The device utilizes Stepper Motors, Step Motor control boards, an Arduino Uno, and a 3D printed Pump frame. Simple code was previously developed using the Arduino IDE. I have expanded on this a bit to allow for more customization (syringe size, mostly). The limitation here, is that the Serial Monitor inputs will be taxing to the users. I need to develop a simple GUI that allows for all of the inputs I need (syringe size, flow (step) rate, start and stop functions, etc.). I need this gui to be a standalone application that can be easily shared (no need to download anything extra, no installations, deploy-able on at LEAST Win and Mac).

Our labs goal is to develop a system that is easy to use and distribute, reducing the costs associated with using Flow Chemistry. I have a decent understanding of Arduino IDE, and I am willing to learn whatever I need to make this work.

I have seen a few things about Python, and 'Processing', but they seem to require the simultaneous use of Arduino IDE which would make deployment too complicated. 

How should I approach this? What language do I need to learn? What advice do you have?
The Uno is connected to the PC with a Serial interface (emulated over USB). The program on the PC only sees the Serial interface. It is not important, if there is really an Arduino at the other end. Nearly every programmung language can be used for interfacing a Serial interface, each gives advantages and disadvantages. But you will not have a single portable program file, that you can start on different OS without installing anything. Either you need to compile the program for each OS individually, or you can use a portable programming language, where you first need to install it's framework on the OS (for example python).

I would propose a different solution (that I'm currently also working on): Depending on your lab situation you could extend your project with a network interface, either Wifi or LAN. Then you can present a webinterface to the user. When writing HTML/javascript websites, it is easy to have them display correctly on most mayor browsers.

That way the GUI is really independent from the OS. For adding the network connection, you could use a LAN shield, Wifi shield, or directly change the microcontroller from an Arduino Uno to an ESP board, which has Wifi build in and is also has more computation power and memory.

The webinterface can be interactive by the use of javascript (since you cannot run php or similar on the Arduino or ESP).

Wether you want to use a companion application, that you write for the PC, or a webinterface is up to you and depends on your exact lab situation.