1 and 2) Guzma is somewhere in the middle, he's given some depth and he has some memorable moments in the game, but he could've used a bit more screentime to himself as appearing at the third island limited the amount of time he had to himself before Lusamine took center stage. I think having his first appearance on Akala, even if it's just a speaking role, would've helped him get more breathing room and time in the spotlight.
Lusamine also could've benefitted from more time, but she really does make the most of what she's given as even before the reveal the scene after beating UB-01 gives an unsettling and threatening feeling and that gets ramped up the more we see of her. Her relationship with her children is also decently handled considering we don't get to see much of their past, and we do get a feel for how an originally loving mother was corrupted by her grief and lost herself to an obsession. In a way, she's kind of like Queen Zeal if we were given more depth to the obsession and the effects of that on a personal level. With Zeal, we get to see the consequences of her actions and how much they affect things on the world stage, but we don't get as much from things such as her relationship with her children, which limits her scope as a villain with a more personal relationship. With Lusamine, we do get to see the effects on the world stage, but a lot of it is more on the personal element as we get more of her past as a good person for a reference point before her fall into madness, we get a better understanding of said fall, and we see how it effects those around her such as Lillie and Gladion. I think those personal elements work to Lusamine's benefit as since we get to know Lillie more as a character, we get to feel more for her plight and want to help her get her mother back from her madness, which makes her place as a villain more unique since there's a hint of tragedy to her situation.
So despite some of her flaws, I'd still rate her very highly among Pokémon's villains, she's a definite threat, but she's given elements that help showcase her transformation into that threat with a little more attention given to the personal element she has with the other characters. I feel that makes her a very interesting character as a result since it's a place that's not as much delved into with the other villains and gives her a neat place among the rest of Pokémon's villainous roster.
3) Lusamine at her most obsessive is pretty insane, she's willing to do anything to be with the object of her obsession even if it hurts her children and the rest of the region in the process, but she's not completely herself either and some of those actions could be excused to an extent. Ghetsis was a cruel and despicable man through and through, he didn't just abuse N, he manipulated him, tormented him all in the quest for power and the effects it had on N definitely left scars that the character had to heal from. Additionally, his treatment of others is just as bad as his Hydreigon has Frustration (which hints at a negative relationship between him and his Pokémon) and Kyurem suffered under him, showing that the depths he was willing to go didn't just extend to N and he was more then willing to hurt others if it met the goals he was trying to achieve. So Ghetsis is still probably the worst of the Pokémon villains in his cruelty, and even Lusamine with her psychological abuse doesn't match him in that regard.