I think volume 6 makes Shima loving Adachi believable, she finally opens up to creating a bond with someone thanks to all the realizations that hit her while thinking about her relationship with Gon.
Throughout the volume she's constantly thinking about Adachi and what her breakdown meant and what their relationship is and how she wants to continue growing it since Adachi is so fucking weird and understands her in a weird level that the moment they get back she can't wait to talk to her, also after Adachi's first extra romantic confession she never feels disgust, or apprehension or something, to me it looks like she's really excited that somebody can love her that much, and especially so if it's Adachi.
Then on the day of the festival and with Adachi stopping fucking around and finally getting shit done (kinda) she does hesitate for a little while since that's just the type of person she is but when she says "sure let's try it" to a romantic relationship ? And with another girl ? Shima's going all out, she has too that's the sort of person she is, I guess she never really thought anybody would love someone boring like her, and she never thought she could love someone so Adachi being all excited about loving her probably is a lot to her and she really loves her back.
