bcool
Deity
You are right, except Isengard is a hybrid city (Primarily Military, Minor Commerce).
Since Isengard is also a Hammer City, it can quickly hammer a Library (4t) and University (5t; Philosophical trait bonus).
Building extra units in Isengard now with give units that will quickly become obsolete, have only 3 XP and each one will cost us 1 Wpt. Building these soon to be obsolete units may make us feel a bit safer and the War Elephants taking a Barbarian city for Diplomatic gain is excellent, but the Longbowmen won't help much in an offensive war, will cost us significant wealth to maintain and while they will probably hold Cimmerian they won't be helping offensively one iota.
If we could build something useful like Trebuchets in Isengard that would be great, but I'd prefer an extra 2 XP via Vassalage or Theocracy or Great Military Instructor. Trebuchets in particular are far more effective against cities using CR2 than CR1 promotion. Also Trebuchets (like all non-Machine Gun siege units) gain only 1 XP per successful barrage engagement, so the extra "training" +2 XP is really import. I would definitely build 3 XP Trebuchets in Isengard, assuming I had no way to boost the XPs.
You have consider the technology and promotion level of the troops being mustered to determine whether to bother with it at all. Build the Library and University in Isengard now while there isn't much of any military unit worth making! It's obvious to me; I'd don't understand why you and bcool are so resistant to the idea; there's nothing sacred about Heroic Epic that it can't be left idle when there aren't any particularly useful military units to build. I caution against trying hard to think of military units to build so you can deny the Minor Commerce city that Isengard is.
Perhaps I should just push through Library + University after the justifiable two War Elephants and one Horse Archer? We can build another Horse Archer for the next Taoist Missionary. Thereafter, maybe the two Horse Archers can cycle fast enough to protect all Taoist Missionaries coming through heading for the West Witches.
Sun Tzu Wu
I believe that we have 3 competing goals that we need to do simultaneously. Balancing them is the key for a good game.
3 goals,
1)teching to Mass Media as quickly as possible
2) designing a military plan to take out the eastern witches to get enough population to win the vote
3) developing our diplomatic relationships with the north and the west, and making sure they don't love the south too much.
I think we are focusing too much on the 1st goal at the expense of the 2nd goal and 3rd goal.
I think the 2 war elephants and the 4 longbows out of isengard will significantly help us achieve the 2nd and the 3rd goal. Because they allow us to capture a barbarian city and defend a city that will shut down Ragnar (allowing us to focus on Asoka when the time comes) and allow us to accumulte the diplo bonuses from the joint war effort earlier than we could otherwise.
The library build in Isengard returns about as much as 100 gold would. But Isengard is well positioned and well suited to build the units we need to help achieve goals 2 and 3.
Please reconsider your support of a library and temple in Isengard.