I wouldn't do the hidden thing (preemptively) unless KDP was definitely dropping individual characters like it was in my case.
And yes, I tested the azk file on my iPad, and the hidden paragraph stayed hidden. (I've never had problems with hiding things before, but I'll be careful to test this in the future.)
In my case, this is the code I used:
<p style="display:none;"><span style="font-family:font1">poiqwenmZWJ,</span><span style="font-family:font2">s</span></p>
I could have used classes, but my defined classes were only for heading styles, so it was quicker just to use style tags.
I don't mind doing a favor to the font designer by not embedding the characters that aren't in the book. (font2 had been properly embedding all used characters except for the lowercase s; font1 had been embedding many fewer characters than it was supposed to.) In fact, I sometimes do this by extracting the embedded font from the PDF of the print version of the book, so the ePub version also does not include the unused characters.