How much can Daeron be blamed for the Trial by Seven at Ashford and its disastrous fallout? To be sure, Aerion was the party most responsible, but had Daeron not been a drunk and a craven and had he not accused Dunk of kidnapping Egg, Aerion would have had no pretext for a trial by seven.

Not really. 

“Aerion has already filled his father’s ear. And Daeron has not helped you either. To excuse his own cowardice, he told my brother that a huge robber knight, chance met on the road, made off with Aegon. I fear you have been cast as this robber knight, ser. In Daeron’s tale, he has spent all these days pursuing you hither and yon, to win back his brother.”

“But Egg will tell him the truth. Aegon, I mean.

“Egg will tell him, I have no doubt,” said Prince Baelor, “but the boy has been known to lie too, as you have good reason to recall. Which son will my brother believe? As for the matter of these puppeteers, by the time Aerion is done twisting the tale it will be high treason. The dragon is the sigil of the royal House. To portray one being slain, sawdust blood spilling from its neck … well, it was doubtless innocent, but it was far from wise. Aerion calls it a veiled attack on House Targaryen, an incitement to revolt. Maekar will likely agree. My brother has a prickly nature, and he has placed all his best hopes on Aerion, since Daeron has been such a grave disappointment to him.” The prince took a sip of wine, then set the goblet aside. “Whatever my brother believes or fails to believe, one truth is beyond dispute. You laid hands upon the blood of the dragon. For that offense, you must be tried, and judged, and punished.”

“Punished?” Dunk did not like the sound of that.

“Aerion would like your head, with or without teeth. He will not have it, I promise you, but I cannot deny him a trial. As my royal father is hundreds of leagues away, my brother and I must sit in judgment of you, along with Lord Ashford, whose domains these are, and Lord Tyrell of Highgarden, his liege lord. The last time a man was found guilty of striking one of royal blood, it was decreed that he should lose the offending hand.”

(Emphasis mine)

Daeron’s self-serving lie didn’t help matters, but Baelor makes it quite clear: Dunk striking Aerion is a separate legal matter from whether or not Dunk kidnapped Aegon, and there would have had to be a trial by combat. 

Making that trial a trial by seven was “my right, I do believe.“ Daeron not having made his contribution would make it somewhat more politically embarrassing for Aerion to claim that right, but he always had the perogative to “insist upon a trial of seven.”