Topmost form, clicking "through" possible?

You can use SetWindowLong to set the WS_EX_TRANSPARENT window style:

If the layered window has the WS_EX_TRANSPARENT extended window style, the shape of the layered window will be ignored and the mouse events will be passed to the other windows underneath the layered window.

CodeProject has this article detailing the technique. Though it's in VB.NET it should be easy to convert to C#.

I have used the following code in the past:

public enum GWL
{
    ExStyle = -20
}

public enum WS_EX
{
    Transparent = 0x20,
    Layered = 0x80000
}

public enum LWA
{
    ColorKey = 0x1,
    Alpha = 0x2
}

[DllImport("user32.dll", EntryPoint = "GetWindowLong")]
public static extern int GetWindowLong(IntPtr hWnd, GWL nIndex);

[DllImport("user32.dll", EntryPoint = "SetWindowLong")]
public static extern int SetWindowLong(IntPtr hWnd, GWL nIndex, int dwNewLong);

[DllImport("user32.dll", EntryPoint = "SetLayeredWindowAttributes")]
public static extern bool SetLayeredWindowAttributes(IntPtr hWnd, int crKey, byte alpha, LWA dwFlags);

protected override void OnShown(EventArgs e)
{
    base.OnShown(e);
    int wl = GetWindowLong(this.Handle, GWL.ExStyle);
    wl = wl | 0x80000 | 0x20;
    SetWindowLong(this.Handle, GWL.ExStyle, wl);
    SetLayeredWindowAttributes(this.Handle, 0, 128, LWA.Alpha);
}

but it also was copied from somewhere else. The important lines here are in the OnShown method. Though I have to admit that the line

wl = wl | 0x80000 | 0x20;

is a little cryptic, setting the WS_EX_LAYERED and WS_EX_TRANSPARENT extended styles.

You can probably also set it like

wl = wl | WS_EX.Layered | WS_EX.Transparent;