@Leinad91
You've got kind of a bizarre way of rating the girls which is fine if that were how you would rate them if Bell was you, but that is just it, he isn't you. For me, I won't ship anyone that goes against the grain of the story if it is core to the plot (like Danmachi), and I place very high priority on the what the character actually feels. I don't like forcing my bias of who my favorite girl is on to the MC if he doesn't have feelings for her. I also think it is very important to take into account character's personality. It has to make sense for the character's feelings to change if said character is already in love. This is especially important with Bell because of his personality, which leads me to your point here:
"However, if Ais, hopefully sooner rather than later, starts giving Bell reasons why she and only her is worth loving, it's okay for him to stay faithful to her."
Bell is a stubborn, one-track/single minded, devoted, and pure person. There has to be a reason for Bell's feeling to change and for him to give up on Ais. We are talking about Bell here who won't give up on someone and who can't go back on his promises, the absolute fool that can't lie to himself. Omori has characterized Bell as someone whose feelings for Ais are strong enough to be the first person to reject Freya in 6-billion years. I know that is because of LF, but the reason he was able to create and maintain it is because of his strong will and purity of his soul.
Ais is the one in Bell's heart and there has to be very strong reason for Bell to start feeling differently given his incredibly loyal and unwavering nature of his feelings. What Ais does or doesn't do changes nothing of the other girls being worthy of love. People usually choose someone because of how they feel. It wasn't that Syr was a bad option or unattractive or unworthy of love, he rejected her because he wants to be with Ais (due to his feelings), promised to chase her, and that he couldn't tell a lie. Ais hasn't given him a reason for his feelings to change, quite the contrary given what happened in 17. I know you discarded it, but I think that is a double standard if you don't consider the events surrounding Ryu's participation in 18 as "plot convenience". The Freya followers "conveniently" forgot to ban any reinforcements, not to mention her double level up that shouldn't have been possible.
If you were going to be kidnapped for sexual harassment and the man you love was in a position to save you (at least try) but ignored your problem, would you still love him in the same way than before? In Bell's case, that was the case, but it's a plot convenience. It's only like that if you consider that to be kidnapped and to raped are trivial things that aren't worth it.
It isn't plot convenience that Bell didn't mind that Ais didn't train him, it is him being in character. He knew that Ais would help if she didn't have her reasons (and don't try to deny she wouldn't have if she was given a free pass like Ryu was to participate by the author). She has after all already demonstrated this time and time again. Bell isn't selfish and he is kind and forgiving to a fault. He just readily forgave Syr/Freya and Hedin for all of the things they put him through. If Bell lost, he would have had to go to Freya familia, but nowhere is it stated that he would be raped. If Freya wanted to rape him, she could have done so in 17...but that isn't what she wanted. She wanted Bell's translucent soul that became hers willingly. She established the sandbox to get him think that his feelings and relationship with Ais wasn't real, just a curse. Very manipulative and pretty horrible, but she didn't rape him. We can only speculate what would happen to him after the war game, but it is still possible that he kept his feelings for Ais and would still try to see her even if Freya tried to forbid it. She isn't going to gain the man she wants if she makes him a slave or forces him to do it, that is exactly what she was trying avoid, unconditional love that she already had in spades.
Anyways, just my two cents. I can already tell you probably won't change your mind, but I do think Bell is 100% in character to stick with Ais despite what happened in 18. Bell is going to be even more understanding if he finds out she was keeping a promise. Bell is one who can't go back on promises either.