^I think it could have been a highlight ONLY if Jon Snow died when he was tricked and the cavalry came crashing on him.
Then again he got saved, because his own cavalry also came crashing on him at the same time
That could have worked, if as with Beric Dondarrion the Lord of Light kept bringing him back to life over and over again as he keeps dying in increasingly stupid ways.
I agree that the Sandsnakes seemed somewhat pointless, except that they were necessary to develop/highlight Ellaria Sand's character. In any case I didn't think they were enough of an issue to diminish season 5 overall.
It was the same for me BTW. I started watching the show during season 3. I binged seasons 1 and 2 and then started watching every week with season 3.
The problem is that they develop/highlight how Ellaria Sand's character is the exact opposite as it is in the books. Book Ellaria has seen too much bloodshed and wants to do whatever is needed to make peace. She recognizes that it was Oberyn's irrational obsession with revenge that led to his death, and believes that no good could come from trying to avenge him. She is content that the death of Gregor from Oberyn's poisoned spear and the death of Tywin from Tyrion's crossbow are plenty. Her focus is on trying to stop the Sand Snakes from ruining their lives and bringing more danger to Dorn in a foolish attempt at gaining vengeance.
The Dornish plot really should have focused on Prince Doran's daughter Arianne and her plot to proclaim Myrcella as the Queen. That would force Cersei into a difficult position, where she must either choose to let Dorne become a free and independent nation or else condemn one of her two remaining children to die a traitor's death.
Dorne was never conquered by the Targaryens. It only joined the rest of the Seven Kingdoms through a treaty and a double marriage alliance. The treaty guaranteed that the Dornish could forever continue to follow the law of the Rhoynar, including the practice of inheritance through strict primogeniture without any preference for males over females.
A strong case could be argued that this means that even if the rest of Westeros considers Tommen to be the heir of King Joffrey, that Rhoynish law and that treaty require the Dornish to consider his elder sister Myrcella to be his heir and the rightful queen over Tommen. The Targaryens had inheritance crises before that treaty where they chose a male over an older female potential heir, but it so happens that after that treaty there was never a case to test that theory as there have been no more female potential heirs with younger brothers all the way down to the supposed children of Robert.
If Cersei decides to side with Tommen, then she must sign Myrcella's death warrant. There would probably be a major war as the Dornish try to press Myrcella's claim and depose Tommen, but no matter who wins one of Cerse's children dies.
If Cersei decides to let Myrcella be the Queen on the Iron Throne, then she is essentially handing over the entire realm to be ruled by House Martell, as Myrcella is an impressionable young girl who is easily manipulated by her consort Trystane Nymeros Martell, the youngest son of Prince Doran. Tyrstane would be sure to have Tommen executed and his head on a spike. Many lords in other regions would likely contest this ruling and rise up in rebellion against Queen Myrcella, perhaps ending in her death too.
If Cersei decides that Rhoynish law only applies in Dorne and that Myrcella may rule there while Tommen keeps the Iron Throne, then the realm is broken and Dorne is free without a fight.
Prince Doran put an end to Arianne's plot because he is a fierce Targaryen loyalist, who intends to see the Lannisters destroyed but knows he does not have the strength to win a war without significant reinforcements from Essos. He signed a secret treaty after Robert's Rebellion in which he betrothed Arianne to Prince Viserys Targaryen, and after his death sends his son Quentyn Martell to Slavers' Bay to try to marry Denaerys and bring her armies and dragons back to Westeros to press her claim. Before he can hear news that Quentyn was rejected and died trying to steal a dragon, he gets news of "Aegon VI" or "Young Griff," another Targaryen Pretender who claims to be the son of Rhaegar Targaryen and Elia Martell, who was believed to have been murdered by Gregor Clegane but claimed to have been saved in a baby swap by Varys. Doran sends Arianne to investigate, and to marry this Aegon if she believes that his claims are true and if she thinks he has the strength needed to reconquer the continent.
He also sends a Sand Snake to take up Oberyn's seat on the Small Council and another disguised as a Septon to infiltrate the High Sparrow's inner circle and guide The Faith to aid their plots against the Lannisters.