1 - Hmm i think that once all her blades where used up the whip like parts of her body were useless, i mean she could have clawed at Cassandra's feced Tentacles but she didnt. I think if she attemtpted to attack Cassandra on her last moments she would be unsucessful and i think hr body was pretty fragile because one of the Trainee Twins managed to slice off one of her fingers. And Roxanne probably didnt want to embarass herslef and look desperate in front of Cassandra especially when she was , as you said, ating calm. Her calmness ws probbly a Facade.
2 - Hmm you may be right but did you notice that the techniques we saw Roxanne acquire were meant to keep the user alive aswell as make their battles easier.
Although Roxanne seems to be an insane Pshycopath when she first see her. I actually think theres a reason shes like that. I think the reason she learned the Blade of Evil from Uranus was because she knew that it had helped Uranus survive against Awakened Beings, and Roxanne may have just been looking out for herself. And when she eventually rose to the Rank of 24 i think that she saw the Blade of Evil as being useless because now that she is a higher Rank why rely on that technique that she may not need anymore, instead of using her own abilities. I also noticed Roxanne was genuinely shocked when she first saw Uranus's technique.
With Neideen her technique was one that would allow her to fight even if her sight was gone, another survival technique. And when she saw Neideen she looked more excited than she was when she saw Uranus fighting. Eventually she would rise to the Rank of No.11 more than ten ranks from her former number.
With Eliabeth we dont really know how she reacted to her fighting but i believe she would have been worse than what she was with Uranus and Neideen. And if i remember correctly the Chapter said that Roxanne "stole" all of Elizabeths Beautiful Sword technique,
making me think she could drain the Yoki of a single warrior, kinda like how she can hide herself from a single oppoent.
And when it came to Cassandra Roxanne looked completely insane. And we al know what happened to Cass

.
So basically i think that at first Roxanne started out as a Warrior who was out to survive. She saw an opportunity in Uranus and took advantage of her technique to maintain her own Survival. As she rose through the Ranks Roxanne probably began to become more confident with herself and began to realize that she didnt have to rely on other for her help, especially if they were weaker than her. I think after the incident with Neideen Roxanne was now focused on gaining power so that she could eventually become the No.1. I think after taking Elizabeth's technique she became more obsessed with becoming the No.1, and when she saw the Dust Eater being used by Cassandra i think that was when she realized that she couldnt defeat Cassandra aswell as that technique being Cassandra's undoing (her unwillingness to use it). So with Roxanne the stronger she got the more crazier she got and the more power hungry she got because in chap 117 it said "Around the time when Roxanne held the position of that Number 5, the position of Number 1 was absolute" the rise in ranks kinda went to her head in my eyes.
3 - Roxanne knew that Cassandra's technique would defeat her even if she did know about it. So letting the No.35 get killed and then letting the Awakened leave unharmed was part of her plan to make her lie about Cassandra rebelling appear to be true. And when she taunted her about her friends death she knew that the other Warriors around her would help her defeat Cassandra by wounding her enough to allow Roxanne to finish her with the Blade of Evil. So yup Cassandra's friends death was what Caused Cassandra to be killed allowing Roxanne to become the No.1 of the Organization
