Both answers to your questions. They are outlined in the manual as noted by Andy Blankertz. Manuals are generally not very good at telling you what you cannot do, but they are generally good at telling you what functionality was created. In this case, you must deduce the fact that you cannot do exactly what you want with an all-in-one button, but you must repeat a few steps.
The only option to download "all images" would be to select each image individually using the instructions on page 142, and repeat as necessary for each batch of 50 images as indicated on page 141.
This may sound tedious, but just to give you an idea of how the Canon software is evolving, the EOS Remote application with which I have to work on my DSLR only allows you to download a single image at a time and does not even allow you to select several images (even less 50).
Page 142 of the SX280 manual:
Page 141 of the SX280 manual: