Vector Software for Drawing and Diagramming
It may be difficult to get all features into one app but I'll suggest 5. They might have features in common but I'll highlight the ones you're asking about.


Artboard - Tablet support  
Sketch - Imports/exports to SVG  
Intaglio - Diagramming  
iVinci - Converts from bitmap to vector  
Pencil - Tablet support (mostly suited for animation and cartoons with bitmap and vector drawing, onion skinning, etc.)


All of them should allow


Editing any component easily after drawing it (layer support)
Easily add and edit text anywhere (vectorized text is a standard)
Exporting to PNG (also a standard)


I can't post more than 2 links but you'll find them easily on MacUpdate (all have trial downloads).