2. You’re right :/
It seems that the size displayed in the website is affected by the width of the image you uploaded.
If the image is very wide the image resizes itself so the that the whole image could be displayed.
That’s actually why all the images are displayed in a variety of sizes.
4. I’m sorry, could you send us a screenshot of the Settings > Reading area so we could get a better idea?
5. Yes, you can edit anything in the theme by editing the code (using the Child Theme of course).
9. Unfortunately there isn’t a built-in way in the Slideshows to resize the images with the grid size.
The images that are larger are re-cropped.