好きになる means fall in love/come to love, but what about 好きになれる? Does it mean "can come to love"? What does 好きになれる mean in this sentence?
If I cannot be with Sempai, then who (else) can I fall in love with?
This なれる is potential. This is a rhetorical question which actually implies "Sempai is the only person I can fall in love with." 誰が好き on its own is ambiguous ("who likes" vs "who does someone like"), but in this case it means "who do I like".
嫌だ on the next page should be just a one-word sentence, "No...", "No way", "I hate this (situation)", etc.