TextEdit Chinese Plain Text Style Problems

I have so far only found one font which has all the characters -- Babelstone Han. Try downloading that and switching the whole text to it.

Another possibility may be the MingLiu series of fonts that come with Word.

Of course plain text does not contain font info, so you are at the mercy of the the app and the OS regarding the fonts chosen, which will never be Babelstone Han unless this is installed and stipulated via rich text. In plain text I think you will always be stuck with two fonts which are unlikely to be identical in their display.

I notice, however, that your text above seems very even as displayed in my Safari. I wonder what font it is using.