The census sheets that we see are the summary ones that the enumerator completed ,so the possibilty for spelling mistakes etc is even greater than you first think.
As you say if they were illiterate then a neighbour may have helped with their household form and by the time the enumerator copied it to his records anyhting could have happened.
So be open to all sorts of different spellings.People who could read and write would also spell a name the way they thought it should be spelt.Those who couldn't read it wouldn't know if it had been spelt correctly or not.
Happens today too

something as simple as my name gets spelt both Carole and Carol....depends if you know someone who spells it with or without an E
Carol
