The reason you could not select the typeface you wanted has to do with the way
the HP LaserJet+ works with characteristics. You changed the typeface to
Helvetica without specifying the "Proportional character spacing"
characteristic. Because this is a high-priority characteristic, and you did
not specifically ask for it, the printer pointed back to the typeface with
"Fixed character spacing," which is Courier.
The escape sequence to make this work is:
ESC(s1P
ESC(s4T
Note that the escape sequence to select Fixed character spacing is ESC(s0P.
Characteristics priority is described in the "HP LaserJet Technical Manual."