4 levels to not trigger the AoO seems excessive, especially since it's the class's primary shtick. 2nd or 3rd would be more reasonable, and I don't think anyone would bat an eyelash if it appeared at 1st because provoking an AoO to make a single melee attack just plain sucks and shouldn't have been in the original Hideous Blow to begin with. Limiting the character to only a single attack serves much better as a balancer between it and the martial characters who depend on full attacks, though I suspect more min-max types will compare that to a rogue and say even a rogue's Greater TWF full attack with SA on each wouldn't be a problem because damage isn't all that people crack it up to be. And both the warlock and rogue get to do some debuffing, though the rogue usually has to spend feats for the Ambush setups.
Dodge, even at +2, can still be fairly crappy if it's against a single opponent. But at least +2 is relevant since it's seen as 10% instead of 1 with 5%. AC needs to get a bigger boost to be relevant in later levels. If you want to keep dodge as a prereq, might I suggest giving the class an AC bonus equal to its Charisma mod? Limit it by class levels like the Duelist does perhaps. Considering the warlock will probably be sticking to light armor unless it invests in the
Battle Caster feat, the extra AC will definitely help it in melee if it has any kind of decent Charisma.
The Improved Dodge feat doesn't seem like it's in any official publications for 3.X. Unless you have a different source than I've found? (Quintessential Fighter seems to have one version, and various wikis have another) Even so, it sounds esoteric enough that you really shouldn't expect us to consider the possibility unless you called it out. If you intended this for your games, then adding your houserules to give some context might work better. Without that, all we have are the regular published rules.
What kind of CR would you assign most goblins and kobolds? A
Warrior 1 kobold is CR 1/4, so combat-capable NPC classes would probably have more HP than can be taken down in 1 hit. Maybe 2, or on a crit. For PC classes, I can see taking them down in one hit happening due to the larger CR compared to their HP.
If the player was made aware of the PrC and then asked "Will I be able to get into this? Can you throw a monster at me that I can defeat on my own to get in?" then I can see a reason for the special requirement. You want to get the player more engaged and interested in their character and the story. However, using a metagame concept such as CR can be a bit of an issue. Do what you will with that information.
I can see at least to possibilities with regards to being limited to CArc + PHB:
1. The DM is limiting books because they don't like the power creep from a lot of splats. Maybe they'll accept homebrew, but chances are they won't because homebrew is seen as notoriously unbalanced.
2. The players are limited by physical copies and would like to have more resources but aren't keen on "obtaining" pdfs. They'll allow homebrew stuff though because it's usually not illegal to homebrew, and since everyone has access to a printer it wouldn't be hard to get a physical copy of the class.
Of those, the first is far more likely. Now, if the second one is your campaign, perhaps you should say so?