It is very confusing for everyone. On that we are agreed.
The dot locator "for mention" is used only in symbols lists or when defining a symbol. So it will be on the Special Symbols page or in a transcriber's note or some other list of SYMBOLS. A key is not a symbols list...it is, essentially, letters (or numbers) that are being used to represent words, so the dot locator is not required. In a pronunciation key, the modified letters are still just letters - they just happen to have modifiers on them. The stress mark IS a symbol and would require the dot locator for mention.
If a transcriber devises a symbol - such as any of the transcriber-defined symbols listed in Section 3 of the UEB code, those are symbols and would require the dot locator for mention preceding them on the Special Symbols page (or in the TN) but the dot locator is NOT required when they are actually used within the text.
The dot locator "for use" is a bit different. It is used when something (like a grade 1 passage indicator) is standing alone or when a symbol could get 'lost' in space (see the examples 3.14).
Does that help?