Xover,
Here's a direct link to the page in the article that has the terrain types table:
http://www.civfanatics.com/doc/civ3/cracker/civ3_starts/opening_plays.htm
This table lists all the most powerful terrain types and displays their RAW DESPOT POWER on the left with the IMPROVED DESPOT POWER on the right if you follow the suggested improvement choice.
If you look carefully at the second column of that table, you will see all the terrain types that already have had their raw food production capacity reduced by 1 becasue of the depot penalty. These are marke with an asterisk.
The terrain types that show a 2 value in that column have not yet incurred the despot food penalty, so if you irrigate them to add one food unit that increase will be absorbed by the despot penalty and you will see no increase.
The terrain types that already show an asterisk and the terrain types that are still held down to a 1, could increase food production without incurring another despot penalty.
It is the wrong conclusion to say you should not irrigate under despotism. Technically it is the wrong conclusion to say you should never irrigate grasslands but in most cases you should not.
Practice the terrain assessment tricks a few times and it will become much easier for you to choose between irrigation or mining.
A good example to file in your memory banks is the choice between two different terrain squares. If you have only one worker and all the squares this worker could move to fall into exactly two groups. If one group is plains that can be irrigated and the other group is grasslands that can be mined; all other factors of position and proximity being the same, what should you do first: Mine the grassland or Irrigate the Plains.
In the opening sequences, the answer is that you should irrigate the plains first if they would make your most powerful squares because it would only cost 8 worker turns (move+irrigate+road) to complete these tasks while the grassland would cost 10 worker turns (move+mine+road) to result in the same power output.
So the answer to your questions 2, 3, and 4 is that irrigation always works, it is the benefit that gets reduced in Despotism in some cases.
Study the terrain table on the linked page above and you wiil get a cleared picture of how the despot penalties impact your improvement choices.
An example of where the mines would not work in despotism would be the Plains with Cattle and this is shown in the third column of the table.
I hope this helps. I am really glad you read the article.
One extra piece of advice I would try to add is that you definately should not fixate on a build order sequence that you think will always work. The build order depends on how much power you have available early in the game and whether that power is concentrated in food, commerce, or shields. I definately emphasize settlers early on, but the sequence of units and improvements should be tailored to the terrain, the civ you have to work with, the difficulty level, and the strategy you have chosen. The same build order should not apply to Japan on grassland when compared to other civs and positions like the Russia on Flood Plains or Egypt in the Mountains.
Good luck,