Why would twilight be shorter at the equator ? ... It seems logical to me that in winter, twilight should be longer and shorter in summer
There's an answer to that
here, and in the final two pages of chapter 8 in Henning Umland's
Short Guide to Celestial Navigation. Umland has neglected refraction (34 arc minutes) and one semi-diameter (16 arc minutes) of the sun, so a better value in lieu of his 24 minutes is 20.7 minutes, i.e. the centre sun travels through a local vertical arc of 5.17 degrees instead of 6 degrees during civil twilight.
A similar derivation is in WM Smart's
Textbook on Spherical Astronomy (art 33, p 51). Corrections for altitude are available in the
UK Air Almanac, p 68 of 2017 edition; see also fig. 3 on p 69 for twilight duration in the Arctic depending on latitude and time of year. Despite the restricted latitudes in those figures the duration of twilight, on the condition of a sunrise existing (viz., upper limb touches the horizon), is shortest near the equinoxes.
Why would [the perihelion] be of any importance when talking about gen nav?
It's of absolutely no importance. Probably an inside joke among Vulcan navigators (à
Le Verrier).
you seem very knowledgeable
I'm afraid that is merely one of those internet mirages.