This may be an unpopular opinion but I personally truly believe Suzaku is superior to Selene in her human form.
I'm going to explain why. Let's analyze their fight.
Chapter 81: Suzaku first attacks Selene with a sword hit but she casually evades hit and counterattacks with her. Suzaku cuts through it rather easily, emerging barely damaged from the attack. She attacks again, and hurts Suzaku a little more than with her previous spell, forcing him to use a named attack to destroy hit. The name "Kurunugi" kind of alarms Selene and distracts her, which allows Suzaku to cut her cheek. Selene asks Suzaku if he really devoured Kurunugi. He confirms it and Selene decides to show him "a bit of her real power".
I think it's safe to say that Selene was starting to take the fight seriously because of how everyone else at the village noticed that the size of the moon had increased. Hakune even commented that someone had forced Selene to get serious which backs up my claims. That last panel of Selene's eye says a lot about how serious she was too.
Chapter 82: Selene starts to overpower Suzaku, throwing attacks of such speed that he cannot even dodge them. He wants to attack her with another attack but she dispels it with hers. Selene finally states that she's going to end their fight with her next attack.
Chapters 82 - 85 are focused on Lucy/Gray battles.
We go back to Suzaku vs Selene at the end of chapter 85, and as we can see, Suzaku's in real struggle.
Chapter 86: We have Selene praising Suzaku's strength, claiming he's one of the strongest humans.
Selene reveals that Kurunugi was one of her sons after Suzaku had asked what their relationship was. She attacks Suzaku and her facial expression is a clear showing of her madness and anger (i.e seriousness) to me, so I believe she really wanted to kill him at the time.
We can even see an attack growing in Selene's hand (perhaps the one she was talking about a few chapter ago?) while Suzaku is preparing to attack. She is shocked as she doesn't recognize Kurunugi's magic, which leads to Suzaku landing his final blow and taking Selene down. She losses a serious amount of blood and even stutters "Impossible... That I [lost] to human... A mere human...". Her reaction to Suzaku's and her facial expression after she took the hit are too credible for me to think she faked her defeat.
A few chapters later, we have Selene who was brought by Suzaku to his guild and is the prisoner of Diabolos. After she goes dragon form and crushes Georg, we have Selene saying that she "appreciates that Suzaku brought her to his guild".
This may be look like a direct implication that Selene lost on purpose but I don't think it's really surprising because it is true that she easily could've used her dragon form to heal her wounds (like she did against Georg) and kill Suzaku in one hit. But she choose not to and let Suzaku bring her to his guild to kill the man who killed her son.
That doesn't really disprove the fact that she's inferior to Suzaku in human form though.
And then we have Selene confirming that Suzaku is stronger than her in human form in chapter 91:
I know some of you guys are going to claim that proof of Suzaku not being on Human Selene's level is that he was helpless against Ignia and Dogramag.
But Ignia has already proven that he was significantly stronger than Selene based on how easily he beat her.
And Dogramag was stated to be the weakest Dragon God a hundred years ago. It's implied that he powered up for the past 100 years, so it's not a stretch to think that he's better than Selene now. Suzaku could cut through Selene's attacks and actually damage but couldn't do the same to Dogramag. Note that Dogramg was made quite indestructible by the the dogra cores and Suzaku had also spent some of his MP against Natsu.
Anyways, that's just my opinion and how I feel. Feel free to quote me if there's some flaws you want to adress.
Honestly, my friend, I really like your post so I don’t have much more to say. Suzaku was indeed stronger than Selene in human form, she lost the duel and even recognized Suzaku’s superiority until she had her primitive form of Dragon to release all her power.
The hype of Suzaku has its beginning was literally titanic : Dragon Slayer of the level able to defeat a Dragon God in 1 vs 1, a demonstration of power reiterated by defeating in an attack, 2 of the most powerful and resistant members of Fairy Tail (Natsu and Erza) literally without forcing. This guy then had a level of Mage unparalleled in history.
The concern is that Mashima does not know how to handle this kind of character becoming too powerful for his own manga and even manages to change and control data from his own story : the passage that you demonstrated and that I adore, has been reinterpreted to give the impression that she was allowed to fall to reach Georg in the sole interest of re-evaluating the too great strength of Suzaku down so that the Dragon Gods remain insurpassable and so that Fairy Tail can defeat "simple" powerful mages and not unbeatable beings putting them no more Dragon Gods' superior level but Gildarts' level. No sense causing confusion to the reader 1.
Mashima even reinvented all of Selene’s personality so that Selene, originally bearer of Chaos on different Planes of Existence, becomes a victim aiming to destroy her fellow beings no longer for her anarchic pleasure but for the Salvation of Humanity. No sense causing confusion to the reader 2.
My God, Mashima even erased the complete history of Luxus' Lacrima originally implanted by his father to strengthen him physically and magically (the latter being very weak at the base) to start on the opposite effect : lacrima held back some of his strength by force ? In what purpose ? To put Luxus at the same level of power as Erza who crushed Misaki’s magic ? If that’s it, it’s so stupid to change the data of a character’s story from his origin to give a power-up whose only interest is to let him be in the most powerful trio in the Guild : frankly, Luxus, who had proven to have a phenomenal increase in power since the 100th Years Quest, has nothing more to prove to anyone. Changing part of his identity as a character is a big mistake for my humble opinion. Especially to match a feat of another S-Class on an opponent of the same level of Magic. If still Luxus and Erza had been rivals throughout the show, why not (although I would not approve that the power up comes from a rewriting of the character and his artifact that gives him his powers) but these two have never been like Natsu and Grey. No sense causing confusion to the reader 3.
I like Fairy Tail but the problem is the inconstancy of his creator who can erase and reshape what he has previously stated with very good bases (like your exemple), to start again on things that no longer hold up the road.