When you put a name in the 'Other Last Names' field they are used during a Search, which is why misspellings should be included. I have searched for those misspellings and found the sources plenty of times where the correct spelling was not found in many sources.
You could put a 'heading' right under "biography' that states 'uncertain Last Name at Birth' and put your explanation there, so it isn't lost further down in Research Notes. I have seen people do that when there are Uncertain parents, etc. You could also put Unknown into your Other Last Names field.