I have Green on one of my fantasy teams actually so I keep up with him. He's a fine player, and a very likable one when you consider his backstory getting back into the league.
Salary wise, you're quite on point with regards to Dunleavy and Green, which I think would be an upgrade (Dunleavy to Green), however, the initial trade was for McDermott who, while unproven, could be better down the line which may be irrelevant to your initial claim but is very relevant in terms of practicality of the trade. Plus getting into the depth question, your depth doesn't really "improve" at SF by swapping McDermott for Green since they're both SFs in this scenario.
And in terms of help on the wing, what do you mean? I assumed defensive help was what you're talking about. To that I ask which team are we matching up against that has a Wade/Lebron like combo? Imo there is no team out there with the exception of San Antonio and maybe Golden State (Iggy and Thompson, but Iggy doesn't score anymore) that has such a dynamic SG/SF tandem that would prove to be too much for Dunleavy or even Mirotic to handle defensively. If you're talking offensive help, then to me that answer is Mirotic thus far as he can do a lot of what Green would theoretically provide plus post up.