[Boromir88]

I think I rather clumsily wrote my thoughts out in the original post.

First I'll start with Bombadil because I agree that he is not an isolationist. I was primarily thinking about Gandalf's comment about Bombadil being a "moss-gatherer." And I failed to realize Morth's point that this makes Bombadil stationary. I think Tolkien has criticisms towards characters who remain static, because Gandalf is the primary good figure and he is a "stone doomed to rolling." Being stationary can lead to isolationism, but not necessarily.

As far as some of my other points...I think I should have began with the different ways isolation turns up. I kind of just spurted out my random thoughts in the first post.

I would say there is political isolation (isolationism, as a policy). This most clearly shows up with Gondolin and Imladris, who chose isolationism as a policy out of an instinct of survival/to outlast the enemy. As mentioned by Pitch and Morth the dangers of this choice (even if it is well-intentioned).

There is also societal isolation, which is found in The Shire. And I would argue this is different than the isolationism of Gondolin, Nargothrond...etc, because hobbits seem to have made the choice to make their society isolated. It's not out of an instinct to survive, or out last an enemy. I actually think the opening chapters Tolkien describes the Shire's isolation idealistically. It may be an ideal but the reality of the world makes the hobbits' isolation unsustainable. As Gildor hints at:

"The wide world is all about you: you can fence yourselves in, but you cannot for ever fence it out"~Three is Company

And then there is personal isolation, this can be seen with Denethor (and there are other examples). But Denethor makes the choice to personally isolate himself, not just in the way he describes arm-chair leadership, but he chooses to cut himself off from everyone. It seems to begin when his wife dies, and then quickly escalates with Boromir's death and Faramir's injury:

'After her death Denethor became more grim and silent than before, and would sit long alone in his tower deep in thought, foreseeing that the assault of Mordor would come in his time.
...Thus pride increased in Denethor together with despair, until he saw in all the deeds of that time only a single combat between the Lord of the White Tower and the Lord of the Barad-dur, and mistrusted all others who resisted Sauron, unless they served himself alone. ~Appendix A - Gondor and the Heirs of Anarion

Perhaps this is the isolation Tolkien is most critical of? To fence your society off from the rest of the world is an ideal, but it's also ignorant. The real world makes it unsustainable. However, to personally isolate leads to despair, hopelessness, and Denethor descends into madness.

I've always thought of Theoden as a foil to Denethor, because they go through similar trials but Theoden escapes from despair in the end. Grima's whispers are isolating Theoden from his kin, and keeping him confined in Meduseld.

"Now, lord' said Gandalf, 'look out upon your land! Breathe the free air again!'~The King of the Golden Hall

Edit: x-post with G55