The missing slope arrow functionality is quite annoying for me also, I really could have used it on my last addon. I did make do and invent a few work-arounds as we Revit programmers often must.
Anyway, regarding your second request regarding arbitary plate roofs, the only case that can't be created using the API is a roof where the vector of maximum slope is not perpendicular to at least one edge, which I am assuming is the exact case that you want 🙂
An idea I've had but not yet tested for solving this case is to insert a minimal chamfer line into one corner of the perimeter curve (as small as Revit will allow) which is perpendicular to the desired slope and use it to control the roof slope. In many cases this would be an acceptable solution and would probably go unnoticed at larger view scales.
If you need any tips on how to acheive this, post a reply and I'll dump some selected code in here.
