Would changing one small detail fix all the flaws in the episode? If Chrysalis were to impersonate Shining Armor instead of Cadance would the episode make absolute perfect sense?
I'm not so sure.
First, imitating Shining Armor would mean Chrysalis would have to figure out how to imitate his shield spell as well, which it is explicitly stated at 8 minutes in only Shining Armor is able to use the spell. So Chrysalis would have to recreate a totally new, unique, super difficult spell.
Second, Shining Armor is captain of the royal guard. Chrysalis would have to learn all the intricate details and protocol of that position (and there are shedloads of intricate protocol involved in the military) to pull off a successful impersonation or else somepony would notice the fake Shining Armor is acting suspicious and an entire military unit would come down on her. Imitating a princess with no known duties prior to the reappearance of the Crystal Empire is a whole lot easier and a whole lot less risky.
Third, if Chrysalis is impersonating Shining Armor, why even have the shield up at all? When she impersonated Cadance the point was to get Shiny (which he will from thus forth be known as) to let his guard down so her horde could invade Canterlot. If she could get rid of Shiny from the beginning - since she is impersonating him - then why go through all the trouble of learning and maintaining the super unique shield spell? Just have the changelings invade right then and end it.
I know what you're thinking: "she has to maintain the deception until Cadance is depowered."
No, she doesn't. Cadance does not know the shield spell. No pony else does. Just invade and then drain Cadance of her love at your leisure.
"But she has to drain Cadance's love so she can defeat Celestia and/or Luna (who didn't do anything anyway)!"
No, she doesn't. Remember, Chrysalis was just as surprised as we were to discover that she had defeated Celestia in combat. That was never an integral part of her plan to begin with, just an unforeseen benefit.
If, on the other hand, Chrysalis impersonates Cadance then she can manipulate Shiny and take down the royal guard without having to know a thing about Canterlot military hierarchy and procedure, and she has the added protection of being thought of as the untouchable niece of the god-empress of all the world rather than just some general who could be court martialed for erratic behaviour.
Changing the story around may make up for some deficits, but it creates a host of new ones that, I think, make the story weaker than the canon version.
Just a little overthinking My Little Pony Friendship is Magic.
