There's so much to unpack here....

Maybe it would be better to just set the whole suitcase on fire and throw it over a cliff. It's seems many people here forget that in the omegaverse, rape is not like it is in real life. Alphas are literally taken over by primal instinct inside of them that is so powerful and demanding that it takes physical restraint to stop them.
Real people don't function like that. Real people don't function like an omega either.
The omegaverse is what many people in the real world have been trying to argue for...well forever--that rape survivors were asking to be raped and that rapists are not to blame and they didn't know what they were doing. The only thing added to this is the idea that once an omega is marked (read bitten) by an alpha they can no longer pursue anyone else. In real like people who are raped are not permanently marked and doomed to never find anyone.
What I am saying is that rape in the Omegaverse is very blurry where the attacker can be just as much a victim as the person who was attacked. In this situation I would think it pretty clear the rapist was not sitting around waiting to attack an omega. He was responding to the omega pheromones just like everyone else. He was in a situation where his natural instincts were not being deterred by others. He was a tool to use in an omega's plot to destroy the life of another omega. He could have easily been replaced by any other alpha who happened to be there.
Yes, I even understand why the story went with Rika falling for his rapist. This is the omegaverse. You can't just crt-z and bite from an alpha. What kind of love life do you expect Rika to lead if he cannot respond to his partner intimately and feels not sense of attraction? It is doomed to fail. It is doomed to fail because that is what the omegaverse is! Omega can't find another partner once they had been bitten by an alpha in heat. Unless the author customizes their version of the omegaverse. I haven't seen any manga or manhwa/Korean webtoons (are there any omegaverse manhua?) that feature this. I saw one where the alpha had died which brings up more questions about omega who outlive their alpha partners and if they can pursue other partners afterward yadda yadda. I digress.
Omega were designed to have very little autonomy of their lives and their bodies. They don't have control over what their bodies do and shouldn't be blamed for it. They are loaded with the burden of stopping their heat and deal with side effects of it. If they become pregnant, they are the ones who must bear all of the burden of raising a child they never asked for. This is hardwired in the very design of the omegaverse.
Was the romance rushed, unnatural and superficial? Yes, yes, yes. This could have been very long (multi-volume) with so much character depth and background building. world building and a deeper understanding of the mangaka's version of the omegaverse. I like longer manga because there is time to develop what you cannot in a singular volume or very short series. Rape is not a topic that can be wrapped up that quickly. It's is soul-crushing, life changing and makes you question, and pull apart nearly every aspect of who you are, your humanity, your right to love, pleasure and dignity. The omega who started the attack, his story could have been much more developed. What are the events in his life that informed this decision as an marginalized person attacking another marginalized person. Yes, he was afraid that the alpha whom he loved would be taken away, but that is not the only reason for the attack. Omega are marginalized. There are deep insecurities at play, inferiority complexes that shaped mindsents and decisions that affect both them and others. It's not an excuse, it's telling a fuller story.
So yes, I agree with everyone who said this fell flat. This fell flatter than the soda I forgot about and ended up drinking the next day even though it said all night outside of the refrigerator. The true tragedy here is that a well-thought out, multi-faceted story critiquing the elements that make of the pillars of the Omegaverse was not written. It could have easily subverted the image of omegas as only being victims and not potential perpetrators of violence as well. It could have opened up a discussion about what is stronger in the end, a fated pair or a pair from marking (biting). There is a lot of disparity in that department. Some affirm that fated pair trumping all and some emphasize choice and autonomy.
Now that I have written a full thesis on this, I will hand this to my non-existent yaoi/bl professor and hopefully continue on to write my dissertation.