For my editing markup I prefer a style that does not overlap and interfere with what might be part of the text I am editing.
Since square brackets indicate comments or amendments in quotes in some editing styles, e.g.
Rogers found in his study that "some [apples] are tasty".
or destroyed text when transcribing manuscripts, I do not use them for anything else.
A question mark in front of the word seems clear in its meaning to me in your context, but as a linguist I have been using the phonetic alphabet, and in this the question mark represents a glottal stop.
For this and other reasons I avoid using characters as inline editing markup.
When I want to mark a word or passage in a software, I change the background color.
I do not change font color, because much of what is published on websites or in print magazines actually has colored text, so this could be misunderstood or overlooked by a second editor (or even myself), but I haven't yet had anything published with a single word or phrase with a colored background.
A colored background seems the most unambiguous markup to me.
Some software, like Word, allows the use of comments:
What I like about this is that you can comment on the reason of your markup, make suggestions etc., but I rarely use these.
If I ever need to mark a word or passage in plain text, I use the number sign:
an elite ###cadre### of sewage treatment workers
I always use three, because a single one might be part of the text, but I have yet to come upon a text with three before and after a word. Also I haven't yet written or edited a text that contained the number sign (but question marks are quite frequent), so they are easy to search (and replace) with grep, e.g.
finds everything between three number signs