In the iPad settings, search for ‘top level’. Go into the customize top level menu settings.
I have a three button mouse on order, the Magic Mouse I have doesn’t have a middle mouse button or scroll wheel to configure. But I have read articles where someone used the accessibility options to configure the middle mouse button. Of course, that doesn’t mean it will work with the web version of SketchUp! I will know more on Wednesday, when the mouse arrives.
I can’t tell you about iPad Pro support, other than it definitely on the list of requested features. I would worry though that if middle mouse button cannot work in the web app, it may not work in any app.