I developed an R package that I would have to license under GPL2 due a few of my dependencies having that license. If I do not distribute this package via CRAN, would it be possible to sell the source code to users if it's for commercial / for profit applications, but provide it for free to academics? If so, how should this be organised in practice? Would it be OK to email the code only to academics, and else ask for a fee? I know that GPL2 would allow any user that obtained the code would then be granted to further distribute the code to anyone else, but I just reasoned that an academic would probably not have much incentive to distribute it to a company, and conversely that a company that paid for it wouldn't like to distribute it further. Or what would the other options be in this case to allow a bit of money to be made from commercial usage of this package, to compensate for some of the development work? Putting the package behind a web interface I heard would also be possible, but that's difficult because of the large files the package works with, which are hard to send over internet. A commercial pretty Java interface to use the R package in a user friendly way I guess would also be possible? I.e. a commercial derivative product? Or would that too have to have a GPL-2 license?
Let me answer your core question here, since you're asking too many other questions for anyone to answer them all:
I developed an R package that I would have to license under GPL2 due a few of my dependencies having that license. If I do not distribute this package via CRAN, would it be possible to sell the source code to users if it's for commercial / for profit applications, but provide it for free to academics?
Short Answer: Probably not.
Long answer: the GPL does not prohibit you from charging whatever you want to distribute code; if you want to put up a website and ask people for money, you can certainly take anything they are willing to pay. It doesn't even have to be your code.
However, if you "have to" license the module under GPL2 because of its dependencies, then the commercial users (as well as the academics) are entitled to demand a full copy of the original source code, and also have the right to redistribute it however they wish. This means that any model you have for charging for the code would be short-lived.
Also, this assumes that you're even distributing the module in binary form to begin with. A lot of R modules are distributed as source, in which case the commercial user would already have the source.