"Why engineer a tuning system for the high end
of the shortwave spectrum to place the LO below the station
frequency?"

One good reason is stability. The high frequency oscillators in these
receivers are less than perfect. So, by using the lower frequency for the
oscillator injection, rather than the upper frequency, you will achieve a
slight, but noticeable improvement in stability.

For example, if you have a 15 MHz receive frequency and a 500 kHz i.f., you
could use either 14.5 MHz as the oscillator frequency or 15.5 MHz. Using
14.5 MHz would yield a 7% inprovement in stability. In addition, the coils
in the oscillator, the tube and the tube socket, like the lower frequency.
One other really minor advantage is that some tubes, particularly the 6C4,
drops off on the high frequency end as emission decreases - so the
oscillator tube might just work a bit longer.

73, Colin K7FM