why did Riley run to her mom? 

simple she was worried her mom got involved in the situation with the news as her mom headed in that direction.

the reason her mom has 18yo woman as her soulmate?

because she is chirpy and very energetic not only that but has a solid idea of what she wants and likes.

another reason is due to Riley's birth, claire (mom) was raped, and for a twist of fate she ended up pregnant, even though she was distraught and grieving due to the situation she decided to keep it as the baby did nothing wrong. ever since she can not see a male in a sexual way turning her into a lesbian, granted it could have made her not like anyone in a sexual way but as the people who helped her were mostly women she ended up trusting them more, to a state where she would rely on women subconsciously compared to men.

the reason Stacy came into the shop and was able to use her finger to take John's head from his shoulders?

she was a victim of abuse. so bad in fact that she went insane, she was hit burned cut you name it she might have experienced it. once the update came the pain in her brain made her come out of her "dead" state and she wished for her gift to be able to blow his head off with a bang, and once she knew she could she went and did it. the joy from being free the relief of not being hurt that way again and of him not being able to do it again washed over her and all she could do was laugh. she then broke for the final time. Stacy eventually ends up in a mental hospital for her own safety. 

why is there so much rape and death so far in the story?

very simple really. the laws of the world are going to be changed in a month and there is the ability to have a superpower the governments are in disarray due to the introduction of the update and being told their laws are not going to matter one month from now, but they upon looking through it found out that only if there is one government for the world can it make a petition for a world law to be added into the world as such they will eventually ether try talking it out or go to war.

but due to that the police in all countries are pulled taught allowing people to do the terrible things they have wanted. and it will only get worse. 

the meaning of this arc?

the reason I decided to add this is to explain what I am doing here. It's to introduce the update to get people to find the one person who will due to their souls being linked love them and be understanding of them granted it's not a no matter what but it's there. some people do not have soulmates others might have hundreds but they might be in a different world.

It also shows that human beings are fundamentally flawed and if given the chance to live how they wish the world will be destroyed until there is a force that will stop it. for instance, look at our world. if there were no laws it would be worse than the purge. but due to people being able to pay others and being able to make laws that the people being paid for would be fine enforcing they were able to control the world.

so this arc is to help our main characters get together to find things they both like and to fall in love. It also shows the world getting destroyed while they are at it.

the reason they were fine once they went home?

because while it is a 10ish minute run it is a 30+ walk they had enough time to calm down a bit and because they were with each other. one with her mom the other two with their soulmates they were able to calm down quite quickly.

the reason sam stayed and was more shellshocked?

sam doesn't have parents they were murdered in front of her as a kid by a deranged man who worked for her dad and tried to take his job and wife, he got so angry he couldn't do anything to get the job and the women he wanted he decided if he wasn't there the job and women would be his. so he broke into their house murdered him and because she didn't want to be raped she fought him and ended up dying. hearing the fight the at the time 12yo sam walked into the room just to see her dad die. she then in shock just stood there while everything else was going on the man after doing what he wanted to then turned around seen her ran hit her and left. he was eventually caught as sams mom managed to not only claw at his eye but also to cause him so much damage he needed the hospital. sam, on the other hand, got sent to a place that could help her and she met her friends there, in the years she lived there she learned as normal and decided to learn about business as there would be no need to fear the situation if she owned the company.