1.) Lambert is on a power trip and at the end of the book really seems to be off kilter compared to earlier in the book.
2.) Also, I think the impression of the number of templars who leave that is given in Asunder is off base.
3.) Templars are chosen from among the most devout correct? So I can see a large number of them saying that they would rather stick with following their religious leader the divine, then follow along with the actions of the Lord Seeker, especially since by and large the templars despise the seekers.
4.) And again, the power of the chantry doesn't lie in the templars or in the mages. Look at the Roman Catholic church. It hasn't had a standing army since probably the days of Pope Julius, yet I believe it is still the largest land owner in the wolrd and is quite powerful.
5.) The Chantry's power lies in faith and its people.
6.) The templar order was never some massive military force capable of fighting wars, the power of the Chantry is stirring up its faithful and motivating its followers.
7.) If the divine truly saw Lambert and such as a threat, she would use her connections to have the great majority of Templars arrested by the countries they are in, in addition she could just brand them herretics, and let the locals deal with them.
For the best interest of all Justinia didn't operate in that manner.
But if you truly think the templars and the circles gave the Chantry its power..... It was a strong power in thedas long before either existed.
I'm sorry if I offend by editing your post to address your arguments by number. It's a long post and I'm lazy.
1. Just like with Anders showing signs of the anger and resentment that drives him mad eventually, Lambert's power trip is evident on Day 1.
2. Most people who are common soldiers follow the paycheck, and if it clears, they're loyal. A majority would follow the organization that provided their livelihood and reason for being.
3. Templars were frequently oblates, given to the Chantry with or without their will. They're also soldiers, trained to fight. Lambert, and potentially the Divine, are out of the picture, so they'll go with their training and fight whoever's threatening them.
4. The power of the Chantry is that it's the Chantry. What else are folks supposed to believe. All they know is that some whackos are fighting and as usual, their fields are the ones getting crushed and their husbands and sons are the ones getting killed. It's hardly a recruitment poster. There's no evidence that the Chantry actually owns land or controls its tenants.
5. The Chantry's power lies in whatever its disciples are willing to feed it. It's the way of life for most folk, but the Dalish, the dwarves, and the qunari differ.
6. Even dumb old Alistair says the templars are an army. Granted, it's a disillusioned army, potentially without a head. With any luck, it will be ripe for the picking.
7. The Divine has to tread carefully with Lambert. By strict Chantry law, he's probably immune to any slings and arrows against him. He might be a scumbag, but he's a righteous scumbag, and he needs to be taken down with more subtle tools than "Because I said so." It doesn't work with kids and there's no reason to think it would work with Lambert supporters either.
As for the last bit, a little romantic, no? People only have power because they get it, one way or another. To find a solution, you have to work within the existing political system. It's like you can have power or invoke change, but not both. People who have it will want to keep it and do whatever it takes to make it so. If it's their lives on the line or yours, it's your job to make sure it's theirs.