Oh, sorry, i didn't realize that this is spoiler for manga readers, but her consciousness is still in Rimuru, and it will be major plot point in the future. The fact that she was absorbed after the death also means almost nothing, since they succesfully ressurected a bunch of monsters days after their death. Granted, there wasn't barrier on Shizu's death, but she was also been absorbed almost instantly after, so it's not like it's required.
P.S. She also commented that her new place is quite comfortable, so you can say that she is living there almost like veldora
About scene of reuniting with mother in afterlife - i don't quite remember if that scene even exist in the original novel, but anyway, you can say that only part of her consciousness was absorbed into Rimuru.