Files are saved out to the documents folder. The user also has the option to save as or export their work via the file system. 

Yes. Sketchable Plus is perfect for importing images for quick edits or use as references. 

Sketchable Document (.sda) will save all layers, color pallet information, and canvas textures. (These files (.sda) can only be read back into Sketchable Plus) .psd preserves layer information. (Sketchable Plus does not read in .psd files) .png, .jpg, .tif, and .dmp (exports flat images)

Yes, you can save an individual brush preset, all brush presets associated with a single tool, and all brush presets for all your tools.

.png, .jpg, .tif, and .bmp