The Hershey Text extension only *adds to* that set of fonts.  The fonts that it contains are “engraving fonts” (by Mr. Hershey) which your PC cannot natively handle, because they are defined by the stroke width, rather than as an outline that gets filled in.  These font types are so different that it is not possible (by any means) to convert an engraving font to or from an outline font.    

So, to answer your questions directly: 

(1) No, Hershey Text can only handle genuine engraving fonts.  

(2) While you cannot print system fonts from within Hershey Text, you are *not* limited to the fonts provided by Hershey Text.  Inkscape handles your system fonts directly (and in many ways better), with the “normal” text tools.

If the text comes into Inkscape as a set of curves (rather than as editable text), you do not need to do anything to it.

If the text comes into Inkscape as editable text (rather than as a set of curves), you’ll need to convert it to curves before plotting.  To do so, first select the text, and then from the Path menu, choose the option “Object to Path.”

Can you say in which way the “Eggbot started to act weird?” 

 A couple of things that you might try: 

 – Import your file into Inkscape as an EPS drawing, rather than SVG. Illustrator SVG sometimes seems to have issues. 

 – Check for extraneous lines or artifacts within Inkscape before printing. From the menu, select View>Zoom>Drawing, to show the full extent of items on your page. Then, Edit > Select All to highlight everything on the page.