To optimise further this symbol, create a rectangle in your 'frame' symbol, pin it to all sides to stretch well. Then place it in the back, and fill it with an image. Set the image to "fill" the frame. Back in your design, you can override the image in your symbols, so you can have different images in your design. When you change the dimensions, the image within will try its best to fill the frame :) cheers.

"Pin to corner" only pins to the nearest corner of each element. Most probably your text element is closer to the right corner and it pins there.

This is a raster image, exported as SVG. You can't do anything about it, unless you find the original vector artwork.

I don't see a point in this. You can always override the text in the symbol with a "space" character.

I guess he used 3px centered border, which would give him 50px, provided the graphic is aligned to the pixel grid.