1) Just a USB CopyNES is needed
2) No cart needed, runs with no cart inserted in the NES. (No video screen needed either, runs without interaction. Just turn on the NES!)
3) Yes - in the coming weeks I am going to do some tests with realtime DMC PCM streaming from the host. I haven't implemented it yet and will take some time to test the maximum stable sample rate. The main goal is to enable pre-loaded samples on the desktop, but eventually it may be possible to stream live audio which would be transcoded in real-time on the host into 7-bit PCM and pushed to the NES DMC channel.
4) Yes - portamento will be done in software (pushing all frequency changes in real time)
Yes, I'm making great effort to ensure it's user-friendly and remains flexible.
Caveats - I'm writing the MIDI driver for Mac only, and it works only with the USB version of CopyNES. I had to settle on this configuration, since my DAW workstations are Macs and there's no parallel port capability on Mac. It's written in Swift so it won't work on PC/Windows or linux without substantial rewriting. The point was to make it as efficient and stable as possible on ONE platform instead of dealing with different platforms and sacrificing stability or features.
I also have a midiNES 1.1. It's fun, but I like mine better
I'm about 75% done - once I'm mostly done all the major features I'll do a YouTube video or something explaining its capabilities and how to use it. And I'll probably release it as a binary at the very least, possibly open-source it at some point.