C# API SDK sends all the params via the single multipart-form item named "json"

OK, I see. I was looking at an earlier version of the client.
Anyhow, please attach a full code sample, as well as the output you're getting.
It would also be helpful to run the units for this client and see if that works correctly.
Note that for the units to run, you need to edit KalturaClientTester/KalturaClientTester.cs and set your own partner ID and admin_secret.