I'm not sure if I understood you correctly, but OpenTK was wiped out and now SDL2 is in use.
Well, you somehow need to give the OpenGL UserControl the graphics context of the SDL_GameWindow. This could be kinda practical when creating an invisible SDL_GameWindow using the SDL_WINDOW_HIDDEN flag.
But you need to modify the source of OpenGL.Net then, because the UserControl automatically creates a context and make it the current one.
It's also possible to create a GL context using SDL:
SDL_GLContext SDL_GL_CreateContext(SDL_Window* window)
But as you can see it requires a SDL_Window as parameter. So you would need to create this SDL_Window (invisible) first and then getting the corresponding context with SDL_GL_GetCurrentContext. But I don't know how to connect this context with the OpenGL.Net UserControl.
I also saw SDL_CreateWindowFrom, were you give the native window handle as a parameter, but didn't manage to get it working right.
I want to mention that I am not an expert at this, but maybe it's a better idea to create the SwapChainRenderTarget.GL by using the SDL_Surface !?
Would be nice to know what someone with a better knowledge thinks about this.