It's good that the atrocities done by the Nazi's were well documented so any evil Empire was created for fiction has to be pigeon holed as NAZISM. Too bad other circumstances hasn't been as recognized such as the British rule, the reign of Joseph Stalin or even the tyranny that still happens in the Middle East where fans can make subjective comparisons.
As for Garak, I thought it was pretty clear if circumstances were different he would and will do anything for the honor of Cardassia. He's not Q, he's a decisive, strategic genius who has thought out every avenue of creating and opposing an immediate threat. It could be Bajorans, Federation, even allies such as Bashir, or DS9, the man is a lethal weapon and I always felt there was an "End game" to everything he's done. This means having an affair with Ziyal and not use it in some fashion to thwart his sworn enemy was completely out of character for Garak. Kira is keen on Garak but I laugh out loud if she thinks he's burdened by her threats dealing with Dukat's half-breed daughter; he's already figured it out.
How unexpected and shocking it could've been if Garak killed Ziyal right in front of Dukat? It would've been mind blowing but with some thought a tiger doesn't change its stripes.
Unlike the Ferengi episodes which were scales of nonsense, scenes between Garak and Dukat or any real Cardassian, not DAMAR, were never waisted in the 1st 3 seasons. Speaking in total subtext and not showing much human formality presented how much they were adversaries, but respected how dangerous they were and wouldn't dare turn their backs on themselves.
Although I felt he was consistent to the characteristics of a Cardassian; Garak was the example of a diabolical mind and there should've been a struggle to examine himself and whether he could exist in a Cardassia which was less diabolical. Unfortunately, after Season 3, the magic was lost from the Cardassian race when the showrunners weren't invested in them anymore.
I agree about your comments considering the Nazi's and how well-documented they are.
But there are certain empires which were as bad and maybe even worse but they're getting away from being as condemned as the Nazi's which is actually a shame. Not to mention that "space Nazi's" has become a cliche in many movies and series. A fictional, future or galactic empire can be as bad or ever worse but it can't be exactly like the Nazi state.
When I wrote a certain story some years ago, I created a dictator which was a combination of Stalin, Hitler, Mao, the Kims in North Korea and a person which I had a conflict with at that point.
(I really regret that last thing because we made peace, became good friends and so, not to mention that the person died a couple of years later which made me feel bad over including that person in the story and I actually changed some things in the story after that. Still I have regrets so I might re-write the story totally in the future, who knows.)
Anyway, one of the persons who read and appreciated the story thought that the fictional dictator must have been modeled after Fidel Castro!
That was a bit weird because I never thought of Castro, when I thought up the scenario, although I have to admit that the fictional dictator may have had some similarities with the Cuban dictator.
(Besides that, most of the characters I make up are actually based on people I've met or know or combinations of some of them, except for the Star Trek characters which are taken from Star Trek!) 
Anyway, when it comes to Garak, I don't think he is the ice cold "lethal weapon" which you desrcibe him as.
No doubt that he is a top-trained secret agent, capable to a lot of things and that he mostly use people he know and certain situations to his own advantage.
We could see that in
Improbable Cause, the episode where he tortured Odo and the following episode
The Die Is Cast and also in the episode
Profit And Loss where he's actually going to hand over Natima Lang hand her two companions to the Cardassians.
But obviously garak did have compassion too, maybe a flaw for an agent of the Obsidian Order but something which makes the character interesting in many ways. When he realized that Tain's plan was futile, he shifted his loyalty back to the federation people and he actually did seem to regret the harm he caused to Odo. The same when it comes to that fantastic twist in the tale when, instead of shooting Quark or natima, he vaporizes Toran, who he obviously hated when he found out that Toran had cheated him.
When it came to Tora Ziyal, I guess that Garak actually had some feelings for her. My theory is that he actually planned to use her in some way to get revenge on Dukat but that changed a bit when he started to have feelings for Tora Ziyal. Maybe he felt sorry for her in some way too. Therefore I think that a scenario where garak would have killed Tora Ziyal in front of Dukat couldn't have happened. Not to mention that a Garak-Ziyal relationship would have insulted and hurt Dukat even more.
But all that is just what makes Garak such a fantastic character!
Good or bad, you never really know!
As for the Ferengi episodes, I did find some of them funny. Not to mentioned that they lightened up Deep Space Nine so the series never became that "dark" series which it's often referred to as being. In fact the, "lighter" episodes here and there prevented that.
As for Quark, he's my second favorite after Garak in DS9.
Also a character with great depth who could surprise the viewers from time to time.
As for Damar, I know that you dislike him and therefore I'm not going ti debate him right now.
But I don't think he was that bad as a character. Not to mention that in my studies of history, I have found many Damar-like persons here and there so I find the character plausible and rather realistic in some ways.
