My thoughts ...
Dynamic Defender
Having to take a feat to change your "charge" for a combat is worth the trade off for versatility. It's not that much of a change to the concept of a devoted defender.
However I would make it a full round action that involves movement to the new "charge". This would stop it being abused by magic that grants extra move actions.
Long Armed Defender
I believe the original class was written with 5' reach in mind, and that is why Harm's Way has the limitation of 5' only. I do not personally see a problem with just modifying Harm's Way instead of making it a new feat. But if you want it controlled, I would take Crothain's advice and allow the feat to increase the 5' limit to reach.
Universal Defender
Although it seems "okay" on the outside, requires Harm's Way and Dynamic Defender, there is one little danger. The Fireball Us Situation. Imagine a Devoted Defender who has evasion and an ungodly Reflex Save or even Improved Evasion. If I remember correctly, the Devoted Defender would be able to take the fireball for everyone around him (max of 8 within 5' reach). I would probably nix this feat.
D