I believe it was a USCCB document that said the responsorial psalm responds to the first reading. It may be true that it does so but I really doubt that’s why it was called responsorial. The responsorial psalm is responsorial, not antiphonal.
Antiphonal recitation or singing means it alternates between two groups. I suppose this could be applied to the Responsorial Psalm, but I think it is more commonly used to apply to two evenly-divided choirs reciting alternate strophes.
The propers were originally antiphonal and, IMHO, should still be but rarely are. The responsorial psalm and gradual were always responsorial.
I agree, but it depends greatly on the cantor to make sure he or she has the proper annunciation so that each and every word can be properly understood when the verses are chanted. Sloppy presentation leads to a boring experience for the listener. It’s like listening to a reader who mumbles, what’s the use of listening at all if you can’t understand what’s being proclaimed? Sure, you can read along but the point of having a lector or a cantor is to hear the Word of God proclaimed.
One of the greatest compliments I receive is when someone tells me that they can really understand me when I sing the psalms. It also saddens me because that means that they can’t understand other cantors.