Rena left on her own accord and now she'll be able to focus a lot more on her mental health and recovery, so I'm sure there'd be no harsh feelings about adding new members. If they're actually adding members from the Hello! Project New Additions thing, I'm sure UFP will add at least 1-3 girls as Kobushi Factory has lost two members within a span of two months. Although, I personally would like for Kobushi Factory to keep the six member line up for 2-4 years and then add new members, much like Juice=Juice after Aina left. I think they're a lot more stable that way.
Anyways, I wish for the best for Rena, I personally know how it feels to struggle with poor mental health, but I'm sure with therapy, a more secure environment and possibly medication, she'll be fine. I'm not going to doubt it was mental health because if it wasn't, they'd probably say it was poor physical health or school if anything.