why did Duncan feel he HAD to give up his claim to the throne in order to marry Jenny of Oldstones? I’m only kinda aware of the rationale behind Morganatic marriages. Why couldn’t Duncan just hold it out until Aegon V died? Would the other nobles just never have acknowledged a commoner queen?

It’s more the latter, plus the nobility are going to be really really unhappy with the idea that their future king broke an oath of betrothal. Kind of a big sign that he’s neither stable nor trustworthy.

And Aegon V could have disinherited Duncan and gone with another son, so he couldn’t have held out. Indeed, his abdication was sort of a way to force a resolution of the scenario by doing that voluntarily and removing any potential question of the legitimacy of his younger brother’s claim.