Yeah, the time travel thing makes me a bit nervous. Aside from the pace of the dialogue, I've liked the overall story, but time travel could potentially be sloppy. Hopefully Chiyomaru pulls through and makes it good.
Radio voice I assume is Ryotasu, mostly because of how much she's been pushing Yuta to go do certain things and she's never around when the voice starts up. Makes me think she pushes him to do stuff and then guided him from afar in a way that wouldn't draw too many initial questions. If she is from the future or an alternate timeline, she would know the stuff Zonko's been eluding to. And my mind could be playing tricks on me, but it also sounds like a vaguely distorted version of her voice. A third party is an option, but I hope it's her.
As for the LN, I really do hope it's just a hiatus. I liked the first LN a lot on J-Novel, and would love to see it continue. But they're developing a VN, the anime will finish and the circumstances around this are similar to the Committee of Antimatter LN indefinite delay and eventual cancellation.
With time travel potentially being at play, I've seen other people theorize that they'll go back to the LNs, they'll have a different ending than the anime and the VN could have both of those endings plus the "True" ending. I think that could be pretty cool.