As in the title. Are there any cases when these two are interchangeable? If yes, when? What are the nuances that distinguish them? It would be great if one can tackle the etymology of the two.
be close to draw near
by side of, beside, near, close
[evening;at dusk (nightfall);toward evening] 靠近晚上的时候。亦称“黄昏”
傍晚 basically is compromised of 傍 "close (to)" + 晚上 "nighttime"
The Chinese literally says night is beginning at nightfall or dinner time.
time after sunset
as to whether they are interchangeable or not, well this really depends on the cognitive abilities of each person - my dusk might be your nighttime - it's hard to say for sure.
as for the entomology of 晚 Outlier says:
日 In 晚, 日 is a semantic component.
免 In 晚 wǎn, 免 miǎn is a sound component, though this is not obvious in modern Mandarin.