Selfishness, even if it is not intentional, is still selfishness.Hypothetically, for her to have enough control of herself to be able to take advantage of the situation, then it wasn't so bad as she made it seem, thus, she was faking.
And no, I don't think she's being selfish. I don't think she did what she did to Jill on purpose, I don't think she's being secretive on purpose and I definitely think she was 100% honest in her talk about James with MC in EP10, she wasn't trying to be vague or had ulterior motives, she just said all she could about the matter before having a meltdown.
She's just damaged and afraid, and because of this she usually makes things a bit more complicated than they need to be (like hiding the rule about dating students at B&R), but she definitely isn't doing it on purpose or because she's selfish and just thinks about her own feelings and not MC's or Jill's for example.
If Bella is fired or suffers serious sanctions because of her relationship with the MC, it will be solely her fault. B&R is not against the romantic relationship between teachers and students if the relationship is public knowledge, but Bella, with her arrogance and selfishness, preferred to lie to the MC about the rulebook. She didn't want to make her relationship with the MC public, but she also didn't want to end the 'relationship' with him. Selfishness, pure selfishness.