Difference between 傍晚 and 晚上 and their etymology
傍

　1. 靠：依山～水。
　2. 临近：～晚。～黑。～亮。
汉英互译
◎ 傍
be close to　　　draw near
English
by side of, beside, near, close

傍晚

[evening;at dusk (nightfall);toward evening] 靠近晚上的时候。亦称“黄昏”

傍晚 basically is compromised of 傍 &quot;close (to)&quot; + 晚上 &quot;nighttime&quot;
晚上

[evening;night] 从日落或晚饭至寝宴的一段时间

The Chinese literally says night is beginning at nightfall or dinner time.
晚

太阳落了的时候

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:

COMPONENTS
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.