Is it completely necessary to replace the hot mirror for full spectrum?

So I have a bit of a filter addiction, and to take full advantage I was thinking about converting an old D90 to full spectrum. If the visible spectrum comes in properly when using external UV/IR blockers, I am even considering having my D800 converted by one of the pros. After quite a bit of research, I am thinking that I may try it myself and remove the D90's hot mirror. From what I have read: if it is not replaced with a pane clear glass or other filter matching the original dimensions and thickness, focus is compromised dramatically as the light will converge at a new point–differentiating from the cameras factory configuration and software settings.

However, I then started to wonder if that is really true for a camera with live view? No matter what the light passes through on the way to the sensor, the camera is still reading that direct pixel information off its surface. So here is the $64,000 question: with contrast detection and live view available, do I really need to replace that filter or can I leave it off, forgo the viewfinder for AF, and use live view? Or is there something I am missing re: how contrast focus works and it does require that extra layer of glass?

The other thing I was thinking is that the filter likely is the only thing protecting the sensor itself, no? In that case I should just chop up an old clear or UV filter on a wet diamond saw to proper dimensions and my theory remains the same... The thickness would be different than the factory hot mirror, meaning focus would still be compromised, but live view should still work fine.

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