Which font is used when I export formulas from Grapher

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.

In Keynote.app: Select all parts of the graphic you need to scale, then copy them to the clipboard (Command-C).

In Preview.app, just create a new document (Command-N). Contents from the clipboard will be automatically inserted. Save this document as PDF somewhere, and drag it to Keynote. This should do the trick.