KL couldn’t be retaken because Aegon would have Margaery amongst others, Tommen taken out or captured  and the seat of political legitimacy. The Tyrells still have an army near KL. I think you’re missing the point, when a rival army was moving to attack KL, he moved to aid KL to the more immediate danger. Also, Euron will likely have been taken out sometime in TWoW.    
Well, GRRM didn’t call it the second Dance of Dragons for nothing. There are bound to be parallels between the two wars, or why give us a detailed description of the Dance? Aegon is trying to form the image of Aegon the Conqueror reborn, I think. Trying to take KL with no navy and a smaller army makes little military sense, especially since there is a Tyrell army of not insignificant size already in the field led by a competent commander. There is also Randyll Tarly to consider who is no pushover. Aegon needs enough military strength to be unrivaled by anyone else in the field.   
Yet, from that description what more could Dany do? Cersei would have already had everything taken from her. I think that is where your argument fails, no offense. Cersei has thicker plot armor than Aegon. 

When a rival army was moving to attack King’s Landing, Tywin moved to save Casterly Rock. It was only when he failed and got word of an alliance in the nick of time that he moved to King’s Landing. 

If Euron is taken out “sometime” that’s not enough time. Garlan is a thousand miles away. And Aegon’s got a 500 mile head start on him. 

He’s already got a parallel – King vs. Queen – he’s not going to literally redo the whole thing. That’s not how GRRM works when it comes to historical analogy – he always remixes. 

As for Aegon – if he gets the Martell army and the Stormlords, he’s got the bigger army. And the Tyrell army isn’t entirely loyal to King Tommen ‘Baratheon’ – remember, these guys all fought for the Targaryens back in the day. And it’s a very good possibility that House Tarly fought for the Blackfyres – their house is right there up against the Red Mountains to defend against the Dornish, a lot of their neighbors fought for the Black Dragon, they have a warrior ethos that would clash with Daeron II’s academic sensibilities, etc. 

Cersei’s plot armor extends no further than not being able to be killed before Jaime shows up again.