First, you need to learn how to make a TCP connection to your EV3. There is a good guide on this here.
Then you need to learn about EV3 VM bytecodes and how to send direct commands.
Since you are using Unity, you may be able to use a pre-existing library such as the MonoBrick Communication Library or the Brian Peek library.
To send a variable, you need to ...
No. Bluetooth on the computer you are using is necessary but you don't need a wifi dongle for either your computer or the ev3. If you are using the EV3+Scratch extension on scratchx.org, all you need is the scratch x plugin and a computer with Bluetooth (or a bluetooth dongle).
Here is a good article about how to set everything up.
According to the dexter website, there is sample robotC code which means, yes you can use the Dexter WIFI sensor with robotC. Maybe there are robotC libraries for it in the download section on that website.
This forum thread also loosley states that you can use the Dexter WIFI sensor with robotC
Hope this helped.