One of the quickest ways to test the master/Mozilla Central build is to use the nightlies.
You can find the versions listed here :
Sometimes the fix is in a version after the build has been made. In this case you can build Gaia on top of that build. There is no need to build gecko in this case. I’ve also done a similar thing when there used to be a ux branch. I would place on the v1-train and then flash the ux branch on top.
For testing the ril, I do compile gecko with a different device. Having said that I compile for the v1-train. I do not compile gecko for the unagi.
[Edit: https://addons.mozilla.org/en-US/firefox/addon/firefox-os-simulator/ is an simulator addon for firefox]
For the VM, I will have to compile the emulator, which I haven’t done yet before. I’m going to try to get that done this week on my own spare time.