Goal

Make the mouse cursor move and click in a different location.
E.g. a smoothed curve or a dragging it along a "rope".

Approach

Intercept low level input events and re-position cursor before any input registers in any application.

Get the event

For this use PInvoke to call the native Win32 API, e.g:
http://www.pinvoke.net/default.aspx/user32/SetWindowsHookEx.html

return SetWindowsHookEx(
    WH_MOUSE_LL, //Hook Location ID
    proc, //a callback to register
    GetModuleHandle(Process.GetCurrentProcess().MainModule.ModuleName), //in what dll the callback can be found, so the current one
    0 //the thread where the procedure shall be called, 0 for all threads
    );

private IntPtr interceptInput(int nCode, IntPtr wParam, IntPtr lParam)
{
    if (nCode == Win32Util.HC_ACTION)
    {
        Win32Util.MSLLHOOKSTRUCT hookStruct = (Win32Util.MSLLHOOKSTRUCT)Marshal.PtrToStructure(lParam, typeof(Win32Util.MSLLHOOKSTRUCT));
    
        //...
    }

    //Recommended by the documentation, pass the event along for other hooks to process it.
    return Win32Util.CallNextHookEx(m_nativeHookPtr, nCode, wParam, lParam); 
}

Modify it

But modifying the MSLLHOOKSTRUCT did not work.
Neither with Marshal back to the pointer nor within unsafe directly modifying it.

hookStruct.pt.x = 960;
Marshal.StructureToPtr(hookStruct, lParam, true);

unsafe
{
    Win32Util.MSLLHOOKSTRUCT* data = (Win32Util.MSLLHOOKSTRUCT*)lParam.ToPointer();
    data->pt.x = 960;
}

All results Google brought up "confirmed" this.
As in, nobody who asked found an answer how to modify the MSLLHOOKSTRUCT before passing it along to CallNextHookEx.
The modification itself seems to work, the object on the native side did change, but the other receivers in the event chain still get the original value.

Drop it

The frequently suggested alternate approach is:

• receive the input event
• apply your logic based on it
• drop the actual event
• emit a new one

When simply setting a new cursor position this works quite well!

if (/*condition*/)
{
    //set the cursor position, this works quite well!
    Win32Util.SetCursorPos(ropeLogic.cursorPoint.X, ropeLogic.cursorPoint.Y);
    return (IntPtr)1; //drop the event
}

But this did not allow to "draw" elsewhere since the mouse_down part of the event is lost.

Re-emit a new one

Emitting an actual event via mouse_event()or SendInput() and thus potentially reacting to your own events smells like potential infinite loops.
(If it happens, restart the PC. taskkill /F and even shutdown didn't help.)

if (/*your conditions, e.g. on LMB_DOWN*/)
{
    //emit a new event, SendInput() is the modern alternative
    Win32Util.mouse_event((uint)Win32Util.MouseEventFlags.LEFTDOWN, ropeLogic.cursorPoint.X, ropeLogic.cursorPoint.Y, 0, 0);

    return (IntPtr)1; //drop the event
}

Check if the event is your own and do not process it

You can check a flag inside MSLLHOOKSTRUCT if its a "injected" event.
https://stackoverflow.com/questions/21928956/how-do-i-modify-keys-in-a-keyboardproc-hooking-procedure

//Check the flag if it's an injected event already
if(hookStruct.flags == (uint)Win32Util.LLHookFLags.LLMHF_INJECTED || hookStruct.flags == (uint)Win32Util.LLHookFLags.LLMHF_LOWER_IL_INJECTED)
{
    return Win32Util.CallNextHookEx(m_nativeHookPtr, nCode, wParam, lParam);
}

This did not help in my case, though it should in theory...