
Was this the real story?
I feel like there were mahor cuts ヽ(`Д´)ノ

Nope. This is the real story! Have fun <3 (jk) http://www.mangago.zone/read-manga/jojo_s_bizarre_adventure_part_3_stardust_crus/

I really still don't get what a mate and thethings about the omegas

I read A LOT of omegaverse, manga and fanfics, so I can clear up any confusion you might have. What exactly don't you get? :)
Mates are ambiguous depending on the author's meaning and role in their stories. Most of the time a mate in Alpha/Beta/Omega au's are like soulmates, someone who is fated to end up with another and there's no two ways around it. Other times, it's just a person they meet, fall in love with, want to spend the rest of their life with, start a family, etc. Most of the time they get married ^o^ and they bite each other on the neck area, a scent gland, to seal the bond and merge their scents together.
Omegas are at the bottom of the A/B/O food chain. They're the one's who (in most cases) face discrimination from the other ranks and are considered good for nothing except breeding. They're typically smaller and weaker than Alpha's and Beta's although some author's don't care for that.
Omega's are the one's that go into heats which is where they are biologically driven to breed/have sex/take an Alpha's knot. They have the urge and need to have sex to make the 'fire' under their skin to die down. Most of the time, when an Omega goes in to heat, they release pheromones that grab the attention of any and all Alpha's (and sometimes Beta's if the author has his/her way). Which is where an Alpha's knot comes in.
Knotting is based on the idea taken from animals, like canines, where during sex the base of the penis inflates, thus locking the Alpha inside the Omega to increase the chances of pregnancy because then the sperm won't spill. It can take hours, sometimes days, for the knot to go down depending on the author/story. Sometimes an Omega will take ANY Alpha's knot and they become lucid and overwhelmed with lust and want to just get some dick. Some author's make it so that when an Omega is already mated, they don't crave or entice any other Alpha except for their mates. Which is good because then the 'rape' tag doesn't come in to play here, thank god. Most Important: Omega's, MALE and FEMALE, can get pregnant. This is where the 'male pregnancy' or 'mpreg' tag comes in which is where a man can get pregnant. Knotting is the sure fire way for an Omega to get pregnant as well.
I hope this cleared some confusion. But if not, here's a link to a person who explains this and other A/B/O things in bigger detail:
http://archiveofourown.org/works/403644/chapters/665489
The 4 story was so sexyyy but that guy was really possessive to the point it was scary (︶︿︶)=凸