Actually this season's finale is going to be packed similarly. Children of the forest, Dany locking the dragons up, Jon becoming LC, Tyrion killing Tywin, Stoneheart.
I don't think we have confirmation of Jon becoming LC this season. Even then, Tyrion killing Tywin and the Children of the Forest are the big scenes there. That's still a lot less than showing an entire coup, Aegon's landing, Varys doing his killings, Jon's assassination, Tyrion's escape from slavery and Dany and Drogon's whole saga in a single episode. Even if you don't show any other characters, you're still going to be reduced down to about five or six minutes for a lot of those scenes. Will Barristan and the Shavepate commit an entire coup in under ten minutes without any buildup since the pit was the previous episode? That seems way, way too fast.
Nine episodes should be enough to complete Cersei's arc. Again, I am basing this off of the way the show has skipped or sped up a variety of things.
Ep 1: Jaime's vigil, Tywin's funeral
Ep 2: Dinner with Kevan, Tommen's Wedding
Ep 3: Beginning of plot to frame Maergery
Ep 4: Cersei orders Davos' execution, introduction of new High Septon
Ep 5: Cersei's arms the Faith. Maergery/Loras inform the court of Iron Born attacks, Loras agrees to attack Storm's End
Ep 6: Cersei is informed Storm's End has fallen, Loras is injured. Maergery plot put into motion
Ep 7: Maergery arrested
Ep 8: Cersei arrested
Ep 9: Walk of shame
Ep 10: Varys epilogue
How is this not feasible? In AFFC, Maergery and Cersei are arrested in the same chapter. The show could easily show that Cersei has been in jail quite awhile before her walk, as the book did.
That's fairly feasible on its own. But you have Cersei appearing in every episode, and a lot of those would require a decent amount of screen time. Not to mention you know they'll want to include the sex scene with Taena, plus some extra small council scenes (they do love those little bits of everyone sitting around for those, and Cersei's should be extra hilarious), and there will probably need to be some extra scenes establishing the faith (since it will be a new plot line) and some time spent with Tommen.
Tyrion in Season 2 is the only example of someone appearing in every episode. They probably aren't going to do it with Cersei next year.
And this comes to the main issue I have.
Could they reach the arrest of Margaery/Cersei in Episode 7? Absolutely. It would require that plot to be in every episode and get a good chunk of screen time, but they could do it.
Could they have Jon's whole Dance arc (possibly plus the LC election in the premier) in one season? Sure.
Could they have Tyrion meeting Illyrio, traveling with Connington/Aegon, getting abducted by Jorah, taken by slavers and then joining the Second Sons outside of Meereen all in one season? Sure.
Could they have all of Dany's chapters in Dance in one season? Sure.
Could they do all of that together in one season, plus Sam's separate journey, Stannis travelling through the North, Theon's story (and if they're ending with the pink letter for Jon they have to get all of Stannis and Theon's stories in), Davos doing his thing, everything in Dorne, Jaime traveling through the Riverlands, Arya's entire arc, Sansa's stuff in the Vale, Connington/Aegon travelling separately after Tyrion is capture and whatever they wind up doing with Bran in one season (this doesn't even factor in any scenes with the Ironborn outside of Theon)?
No, I don't think they can. There's just not anywhere near enough time to do it all. Some characters will have it all (Sam, Davos and Sansa will almost certainly finish up their Feast/Dance material by the end of Season 5), but unless they're expanding the episode lengths, there's no way to fit all of those arcs in the season. You can't have five or six different arcs which all require as much time as Tyrion's Season 2 arc.
I don't see how Jon's chapters could be split into two seasons. What's the halfway point/cliff hanger for him?
I see Tormund and Jon reaching their deal as a good stopping point for Jon in Season 5. They could then get to the assassination around the mid point of Season 6, leaving the fallout and first half or so of whatever happens at the Wall in Winds for the second half of Season 6.