Okay. Same text, new question. Should the "when" be left before the side comment or moved to after?
The "when" belongs where it is, for the same reason why you should use em-dashes instead of parentheses: it's interrupted speech. In this case, the character is interrupting their own thoughts, demonstrating the chaotic nature of their mind at the time. Em-dashes are specifically designed for interrupted speech, but also do double duty as separators for sentence fragments which aren't directly related to the sentence. In your case, they're doing both, so your only real alternative, if you maintain the current sentence structure, is to use the em-dash (plus, you rarely see the parenthesis in fiction). You'll notice I use them all the time on the forum or in emails, but never in my stories.
I find it a useful means of introducing a new topic or as a gambit for reopening a prior topic's discussion; said topic to be discussed sometime after the current topic is concluded.
Yep, that's another great use of them. In that case, you're foreshadowing a future conversation between the characters in a natural manner which doesn't seem forced.
One reason the parentheses aren't used in fiction as they're generally interpreted as 'author intervention', the author explaining the story directly to the reader, bypassing the characters. Readers tend (general assertion warning) to see the punctuation as non-native to the speaker.
As my description of the interruption of speech shows, these interruptions can mean very much. Displaying it as an interruption in someone's speech helps to show their mental state, while the parentheses tend to be interrupted (correctly or not) as an external aside by the author himself).
Ernest's method, moving the aside to a private conversation with someone else, works as well, but requires scheduling private moments between the other parties, which often necessitates juggling scenes around--something which isn't always easy to arrange.