I posted this in the other thread but I think the showrunners are expecting us to put two and two together more, partly because they don't have time to fit in as much since they are rushing through these episodes and especially since we don't have the books to fill in in the gaps. In Euron's case, if we take him literally that he has a fleet of over 1,000 ships, he could have easily split his forces and sent half to the Rock and half to smash Yara's fleet. In Highgarden's case, I think that when Tarly flipped he would have taken most, if not all of the Tyrell bannermen with him. He could have easily kept the news that the Lannister army was marching there a secret and caught Highgarden by surprise. I would liken it to similar to how Roose sabotaged Robb's forces without it being readily apparent.
However I definitely agree about the Battle of the Bastards, that was just dramatic tv cliché to have the Eerie forces show up at the perfect time with no warning. Also the attack on Yara's fleet was very tv, if that occurred in the book somehow, I think Martin would just write it as the Iron Fleet surprised Dany's ships on their way to Dorne and destroyed them without many more details than that.
If the show is following some of the same arcs that GRRM was planning on putting into his books, no doubt it will happen differently or a little more plausibly. For one thing, I wouldn't think the Tarly depicted in the books would flip, he is an #######, but he's also a soldier through and through. In the books the Iron Fleet is already at war with the Reach and the Highgarden forces are already taking losses (plus there is a male heir in Highgarden in Willas) so who knows how it plays out in any (possible) future books.