vesper < *wo-kWspero?

There's a lot of words for 'night, evening, darkness, cloud, fog, smoke' that seem to be linked but their correct relationship is not well-understood:

Sanskrit. ks.ap -
Avestic ks^ap- ks^apar ks^afn-
Latin vesper, creper (cf. crepusculus)
Greek hesperos, knephas, psephas, dnophos, zophos
OIrl fescor
Lit. va~karas
Arm. gis^er
Hittite ispant-

Taking Avestan, Irish and Hittite as related we can see an heteroclytic word: *n/r-, so ks^afn-/ks^apar could imply in a PIE *ksepr-/ksepn-
*wesperos (Latin, Greek, Irish, Armenian) would be *we-kWsp-er-o .
Greek words doesn't fit in regular IE shifts

dnophos : zophos < Pre-Greek *dn^opHo-
knephas:psephas < *Pre-Greek *knepha- / *kWsepha- (= ksap?)
or psephas < *kWtyephas < *tkWn^epHa- ?
knephas < *tkwnephas- < *tkWn^epHa- ?

JS Lopes