Would've made more sense to rename Osha.
No it wouldn't, because Osha has already appeared in the series. It's far too late to rename her.
Yes, but presumably the writers knew of Asha's existence prior to writing the first season, and knew that Asha was going to be significantly more prominent than Osha. Would've made more sense to call her something else had they been planning ahead...
As for Alton Lannister/Cleos Frey, I have no idea why the rename, particularly from Cleos to Alton. Of course, there are so many Freys to keep straight anyway that they may have decided that fewer Freys to keep track of may have been better over the long run. (Somewhere on the Internet, someone actually worked out the line of succession for the entire Frey house, and it's insane.)