If you go to your Demigod folder, open dgdata.zip, and go to the units folder, you can find the information. The files you want are HighPriest01_unit.bp, etc. Just look for the unit.bp ending. You can open them with notepad or something similar. For the Demigods, most of the useful information is towards the end.
You can see all the unit info here , though I'm not sure how up to date it is. I looked around for lua files, but didn't find anything useful. You can open them with notepad or some other text editor if you want to take a look at a few yourself.
It is possible that they don't stack how you are thinking. I don't know how it works for sure, but this matched up very nicely w/ your numbers. For fire nova, -20% would be 12. -15% off of 12 is 10.2. Another -15% off 10.2 is 8.67. If you are using the number on the skill when it is red as the "actual cooldown" it would round up to 9 in this case, but only stay like that for .67 seconds. I only did the math for a few, but it seems right.
I believe that the AI is meant to be weak. They are meant as a way to ease new players into multiplayer. The plan is that as you play more games and get better, AI will be removed. It isn't doing that yet because of the stat issues.
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I don't see how consumables are OP. I never use them and seem to do alright. Game I just played I had ~15 kills, 3-4 deaths and was the only one to get to lvl 20 (5v5 w/ 2 humans each, and enemy players did use potions).