There are several problems and I don't want to confuse all.

First problem : the character "?" :
So can you tell me if I was right when I told you that you must change your font in your text viewer to make "?" disappear?
More precisely: set CP to 1252 and write "Œcie¿ka" in TYPViewer and save to txt. (exactly like you did in your previous post)
Then open your txt with Notepad or other text editor and try different fonts and tell me what you get.

I do think that the characters"?" you get is a replacement character because the character the system must display doesn't exist in the font.
It is not a problem of charset, the character 0xBF is not converted in 0x3F.
The character 0x3F doesn't exist in all fonts. So , if you try to display it in a font in which it doesn't exist, the system (Windows) replace it by a "character of replacement" which is "?" in your system...
Please confirm or infirm that.