Depth of Field relates to camera taking lens and the circle of confusion is much larger in a camera taking lens then that of a flat field lens.
Depth of Focus relates to flat field lenses and is very small.
When enlarging we are using a flat field lens and the principles of depth of field do not come in play.
I would admit to notice a very , very minimal existance of depth of focus when projecting a negative but not enough to sharpen images at the edges.
A properly aligned easel board>lens>negative position> condensors> and light source are the considerations for sharpness.
A glass carrier is mandatory for my work. If I was only working with 4x5 negatives *thicker base* then I would consider glassless carrier.
for 35mm, medium format, polaroid negative a glass carrier is mandatory to keep the film flat through, longer exposures, split printing and complicated burn patterns. * the negative will rise in the middle towards the bulb with heat, which will cause pop negatives*
This is the case in my work and the pain of cleaning the glass is minimal compared to the suriety of sharp film along the whole print.
Others may have found a cure for popped negs other than glass, I have not.