Also probably makes sense to only clock idle time when a player is below max pop.  Anyways, these are cool graphs.

What should they build? The AI uses the building classes to know what to build next.

Depends, on what your OS is. It's usually next to your pyrogenesis executable.

You could. Basically you have a setting to customize the classes that are included. Since this use the trainer's functionalities, you can use its "Training Entity" setting in options. I've just realized that means, for the experiment branch i pushed you need to have the trainer option on for it to work too.  (you can still toggle it off from the gui obviously and works when replaying, just need to have the option on for technical reasons).

Maybe for a better perspective might be cool to exclude this

The % are high, but you can see on the graphs that it's a lot of it that come from when the game is about to end when often some players are full pop and some have no resources at all to continue producing . As well the buildings taken into consideration are CCs and Military buildings: so forges, workshops, forts are taken into consideration if they aren't researching or training. (towers etc DON'T have the Military class so it's only really the ones listed above).