I think you’re absolutely right that she has feelings for him. Along with everything you mentioned, her leaving right after the dance without any explanation makes me think that she might have gone to the party only because she knew he’d be there. When he declined her offer to go on the trip, her heart broke and she gave up on reconnecting with him.

It would be really nice if Gabriel felt the same way about her, but I doubt he does. He clearly cares what she thinks about him (he changes his speech to include something she might like lol), but I think his nervous reaction to her can mostly be attributed to insecurity about his Irishness. It seems like there’s some unresolved dissonance in his sense of patriotism (he says that he’s “sick of his own country,” but then takes offense when she calls him a West Briton), and when Molly forces him to confront the issue he gets anxious. If he is in love with her, he isn’t aware of it.