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Just an FYI of what I did to get usernames with spaces working in mentionslookup.
This of course assumes you're using double-quotes as your mentions+ delimiter; you would have to change it based on how you set up mentions+. Hacky, but works (seems to be my motto around here - I just barely have time to put out fires, let alone work on plugins ).
I think in my version of mentions+ (0.12), I did this or something like it on lines 386 (inputCaretInsertMention) and 402 (frameCaretInsertMention). There may be other places I should be editing, but it seems to work so far.