Is it possible to redefine what Keynote interprets as bold and normal for a particular font?
To answer directly to your two questions: 

a) No, you can’t redefine what is bold and what is normal as far as I know, Medium and Light are different styles than Regular and Bold and there isn’t a keyboard shortcut to set them (nor a menu item for that) and because of the latter, you can’t set a keyboard shortcut. 

b) Because of (a), you can’t really do this.

A (lame) alternative is to use Copy Style cmd+alt+c and paste style cmd+alt+v. But that requires that you find a piece of text that has the same size/font.

Also having the Formatting bar visible makes it easier (albeit with the mouse) to change that: shift+cmd+R.