If you can live with having to distribute Csound with your plugins then why not. Csound is a powerful language. There already exists quite a few commercial applications that use Csound. Here are a few ones I can recall right now..
If you mean hide your Csound code from the user, then the answer is not that clear. You could modify the source code of Cabbage so that it bundles the csd's into the binary executable, but then anyone who understands C++ could rewrite things to expose the source code. I don't think you need to worry. Many people who use Csound will be able to tell how you've programmed your instruments without needing to see the source code. And those who don't probably won't even be able to understand your code to begin with
If you mean convert the Csound code into C++, no it's not possible. But you could write a low level C++ processing algorithm and use it instead of Csound with Cabbage, but you would need to public the source code or else you would be in breach of the GPL.
Cabbage 'should' run on any DAW that supports VSTs.
Over the years others have helped with developing Cabbage, and continue to do so today. Fork the branch and submit pull requests as you see fit. I've just rewritten the source code, which I'll release soon. That should help others to better understand how it all works. It's a lot leaner now than before. For what it's worth, I use Cabbage to make my own music. I've been playing since I was a small child and I don't plan on stopping any time soon! Regardless of my involvement, the project is open source. So anyone can chip in.
I'm pretty sure there are if some of their questions are anything to do by. In fact, one member posted recently about a new commercial plugin they have developed with Cabbage. I see Csound and Cabbage as a great prototyping tool. You can mock up a plugin with relative ease. If you think it's got teeth you can either try to package it and distribute it, or see about porting it to C/C++. The second option will involve more time and effort, but you can avoid having to adhere to the GPL. But the first option is quicker, and may free up more time for marketing, website, distribution etc.
Hope my answers make things a little clearer. Feel free to probe further.