I LOVE it. Frankly, I have no idea about how to explain why there is no "-s" on "come". I am guessing that it is some kind of relic from a "future subjunctive". There is such a mood in Portuguese and it changes the verb.
As far as the diagram is concerned, I would simply treat "When" as understood. "If" might be better to be understood because it, at least, would call for the conditional mood.
That's a good one!
- For Teachers