Alloromantic graysexual here.
I don't know how to answer the "how do allos manage" question. I guess it's because I'm alloromantic myself, but experiencing romantic attraction doesn't sound that harsh to me lol not more than feeling alterous or platonic attraction!
"How come you can't just not-be-in-love with someone that doesn't like you back?"
Ohh this is a good question. I actually think you can, most of the times, but maybe not straight away. I think what you want plays a role, but it can also be very random. And very hard. When you feel a certain type of attraction for someone, it means something in them makes you feel it. That something may be there even if they don't like you back, so most of the times you can't simply say "okay I'm not going to be romantically attracted to you from now on". Sometimes attractions and feelings change over time and you can move on - let me add: if you want. I personally don't think it's always "needed" to move on, contrary to what we are constantly told, namely that if we can't have a romantic relationship, then we have to forget our feelings or we will suffer o_o Personally, I think I felt romantic attraction for some friends of mine and I was completely ok with the friendship we had O_O
Back to your question, let me make two examples.
Can people always just consciously stop being sexually attracted to people? I don't think so. Some allosexual friends I have even felt sexual attraction to people they found unpleasant, sometimes. And sometimes they couldn't just say "yeah ok from now on I'm not going to be sexually attracted to you".
I don't know if you personally feel alterous or platonic attraction, but... Can you always stop feeling that on command? Of course when people hurt us sometimes our platonic or alterous feelings tend to fade. But I think it's not always the case.
In the past I was reaaaally hurt by people I had strong platonic/alterous feelings for. 0% romantic feelings. But at a point, they got fed up with me and didn't want to be friends anymore. It hurt because I couldn't stop platonically loving them right away. I still loved them, and it took time.
I never experienced anything like that with romantic attraction, personally, but I guess it can be similar :) Although I personally (personal experience here! I know many, if not most, alloromantics are not like this) find it much easier to "move on" from romanic attraction than platonic or alterous one.
"Is kissing someone you're romantically attracted to different than someone you aren't?"
... I was never able to kiss someone I'm not romantically attracted to, due to some circumstances, so I can't answer, sorry!
"How can you know that someone is The One?"
I think "The One" rhetoric is simply toxic amatonormativity. There's no such thing. Some people believe it and I respect that, but I guess what they feel is not that much different than what I feel: normally I simply feel the person I have a relationship with is a person I like being with, I love them and we make eachother happier being in that relationship. That's it, I guess. I don't think they're the only person in the world I might love romantically or the only one I could like being with and who could make me happier in a romantic relationship.
I'm not sure I answered decently, but well, I tried.
Feel free to ask anything else :)