Ok, first off Sephiroth didn't become more "Cetra-like." He was just entombed in Mako.
Next, remember why everyone was at the North Crater to begin with? The Reunion! Jenova wanted her cells back. So, she took the Sephiroth clone and, I assume, absorbed her cells back. She must not have done that to Sephiroth because:
a. She still had some use for him, or
b. The cells somehow mutated with Sephiroth, which would also explain why she didn't absorb Cloud.
