How to fix and/or replace cracked glass?

Oh boy! What a pickle of a mess!

Sadly, the way Apple constructed the display makes replacement of the front glass very close to impossible.

Getting the display assembly off follows the same process as the newer iMac’s using the pizza cutter, then once you get the assembly off, you next need to take the backlight unit off and then the defuser sheets. Next is separating the LCD panel from the cover glass this is where the risk of damage to the LCD panel as like the iMacs there is a gummy foam seal that holds the two together. I have yet to see anyone either get the panel off without damage or leaving a mess on the glass or the fragile LCD panel which is very difficult to remove.

While very expensive you really will need to let Apple replace the screen assembly as there is a needed calibration procedure to make the zone backlight to work properly.

You can’t.

Apart from the extremely risky taking apart process and not able to get parts, you’ll need a bonding and debubble machine as big as that giant display, which nobody makes for repair shops.

Industrial versions that are used in the display factories exist, but that thing is the size of a CT scanner and probably cost as much as a new car.