OK. Trying to please every e-reader around is enough to literally drive a man crazy. I spent way too much time in this &%!&% ePub and every time I manage to get something working I seem to break something else.
The Apple symbol. Yes, it works in Sigil: I set "height: 1em;" and the symbol is the size I want (just as Text2002ans showed). But as soon as I open the ePub in ADE the Apple gets back to its giant size. And frankly, I have no idea why.
The most confusing thing is that when I open Text2002ans's AppleTest.epub in ADE, it works just fine! The Apple symbol is shown at the good size. So I tried to import his exact code in my ePub, same PNG file, copied the CSS and HTML.
And it doesn't work anymore! Giant Apple in ADE again...
Frankly, this is getting insane. I really have no idea what is causing this. At this point I'm a little desperate. I spent way too much time trying to get this ePub to work this week...
Even my other embedded symbols seem to have stopped working in ADE.

And my 'Special-character.epub' test file posted on the first page still works fine in ADE, and the fonts are embedded
exactly the same way.
I can't upload the ePub here because of copyright, but if anyone is curious about this and wants to have a look at it, let me know and I'll send a link in private.
Thanks again for all the help.
EDIT: Oh and by the way, the AppleTest.epub (with the PNG file) doesn't work in iBooks either. I get giant apples there too... The most frustrating thing is that iBooks is pretty much the only reader where I wouldn't even need to do this: Apple symbols are already embedded in it so the symbol appeared automatically on the first try...