Again, I feel it is absolutely imperative you attempt this before trying to find fault. Consider that, on Monarch, the cost to support nine cities based on number is 9gpt - a tally that can be satisfied with two merchants. (Or, more to the point, the cost of 9 cities in maintenance can be paid for with one of them leaving the other eight cities free to do as they please.)
I quite agree that you should always attempt this when arguing this case. As a result I've spent a while messing around in game and in worldbuilder running some tests on the ICS strategy at monarch level. The numbers you present in your posts would make it a strong, quite possibly the strongest, strategy, but there's a snag:
They bear no resemblance to the numbers in the game.
Firstly your core block of nine cities. With all cities at size one this doesn't cost 9gpt, it costs 37gpt. This breaks down as 4gpt from distance, 20 from city number and 13 from civic upkeep (which increases with number of cities, so also has to be factored in. All these tests use the default low cost civics). When they reach their optimum size of eight (using all available squares), they have a cost of 70gpt, breaking down as 8 from distance 27 from city number and 35 from civic upkeep.
Adding the second block around the forbidden palace the costs get even nastier. For the full 18 size 8 cities (maximum tile usage) it costs 205gpt, breaking down as 16gpt distance, 108 city number and 85 civic upkeep.
Take the comparison between the first block and my 4 large cities. At size 1 they cost me 10gpt (breaking down as 3 distance, 4 city number and 3 civic upkeep). With 4 at size 20 it comes to 49gpt (breaking down as 13 distance, 8 city number and 28 civic upkeep).
Now I'm not saying ICS is completely unviable, but it is much more expensive than you make out. Compare the first block of 9 to my 4 large cities. I can use 80 tiles, plus the 4 weaker city tiles. Your block of 9 can use 72, plus the 9 weaker city tiles. As you expand to a second block and further the maintenance costs increase even faster. Now for a more general list of pros and cons for ICS.
Pros
Easier to defend as cities are easily reinforced and all tile improvements are easily defendable.
Boosts effect of Mercantilism and Statue of Liberty (how are you going to build this under ICS by the way?)
Can use tiles earlier due to faster growth at small city sizes
No health or happiness issues
Cons
No powerhouse cities, so much of the bonus from national wonders is lost.
Little chance of getting Middle ages and later wonders
Major loss of trade route income, which increases with city size, and in any case you have little choice but to run Mercantilism.
You have to build twice as many city improvements, many of which will be slower due to the small city sizes.
Fewer great people due to aforementioned lack of wonders, GP farms etc.
You've had to build more settlers.
Now do you see why I find ICS is a weak strategy? You can reduce the city maintenance to some extent with courthouses and the organized trait, but the same is true for a large city strategy. Mercantilism and the Statue of Liberty (assuming you get it) will help ICS catch up a bit, but they barely compensate for the lost commerce with the 9 city block, and they won't bridge the gap for an 18 city double block. On top of that you have the penalties listed above. I might consider it in an always war game, where the boost to defense is more valuable, and Mercantilism is a given anyway, but not in a normal game. The designers set out specifically to kill the ICS strategy in Civ 4, and it looks to me like they did a pretty good job.