WP8.1 is planned to be announced in April at BUILD. We're unsure if the developers preview will become available then or not, though it seems likely. There's some information that current customers may receive WP8.1 soon after that but not sure how reliable that information is.
I don't think the carriers 'tinker' with WP that much unless MS plans to change to allow that. The OS is the way it is on all phones. There are some 'features' that get turned on or off depending on country and carrier preference.
Until WP8.1 is released there won't be any ROMs available that you can flash to your phone and even then it may or may not be stable though there are a few success stories.
Not sure if it will be the same but with GDR3 we just had to sign up for Windows Apps Studio:
Windows Phone App Studio - Microsoft