@X-CNC
We did some time ago a CNC router with the glass tube mounted on the spindle. It is a big machine so it was not a problem. But most of the answers you already have from @LarryM and @anon68752607. I would add here also some other points:
- Diode lasers are small and there is no need to make the weigth of the moving part smaller, so usually it is just mounted on the moving part.
- Beam profile - as the CO2 laser have really nice round beam with quite small divergence in the diode lasers in order to obtain higher power you need to use lens with high NA (numerical aperture) so for example G2 lens. Nevertheless this kind of lens are producing the beam with very bad divergence, for example around 6mrad. This kind of beam at 5 meter distance has 25mm size! This means that to focus the beam you will need the lens with diameter of 25mm. The longer the distance the bigger the beamspot. This kind of lens are not easily available since there are mostly two types of lens (cheap ones) one are 25mm for CO2 and second one are 5-10mm for diode lasers. So just for long distance it is hard to produce cheap mirror + focusing lens setup. Anyway with such machine as X-Carve it is possible since maximum length will be around 2,5 meters. There will be small problem with cutting because of the light cone angle but it is possible to make such a setup.
What is the margin of error when focusing a diode laser? In other words, how far off the ideal focus distance would a laser have to move to see degradation of quality? I’m asking because I’m wondering if something that was, say, .020" bowed/warped/out-of-flat would affect the quality.
This is tricky question 
If I understood correctly you speek about focus depth.
There are equations which allows to calculate it but they are not very simple.
In optics and especially laser science, the Rayleigh length or Rayleigh range is the distance along the propagation direction of a beam from the waist to the place where the area of the cross section is doubled.
Here you can find some equations:
Overall:
When you focus the beam at 100mm then focus depth will be longer than when focusing the beam at 30mm distance.
Here are our measurements.
Beam was focused at 100mm with beamsot size around 0,5mm.
Each line differs by 1mm in Z distance.
Each dot differs by 0,1mm in Z distance.
As you can see the smallest dot do not change with Z change in around 1,5 - 2mm. To obtain square dot Z can not change by more than ±0,5mm. Anyway this is also effect of burning because even with 0,1 beam spot you can make a burnout which is 0,2mm. With special equipment you will be able to measure it more acurrately but for engraving and cutting it is not that important as you see.
The measurements we did are for long focus and big beamspot (just to see it better). Anyway you asked about 0,02" which is close to 0,5mm so exactly the size of the beamspot you see on the picture.