Please stop saying that Shining Armor is Deus Ex Machina. He isn't Deus Ex Machina. If anything he is a plot device.

For him to be Deus Ex Machina there would have had to be a situation when he was the only way to solve a problem. If Twilight had been told it was Cadence's wedding and that Shining Armor had come out of(literal) nowhere to help her defeat the Changeling Queen with the power of love. Then revealed that it was him she was marrying then you could call him Deus Ex Machina. So at most he is a Plot device but I choose to see it as something we never really needed to know. They could have mentioned him or shown him before but they either chose not to or just never created him. That is why if they made him just for the episode he was just a device to make this plot happen a "plot device" if you will. I'm going to stop saying that now.

As for how the Changeling Queen got into Canterlot I figured that she probably snuck in hidden after the wedding announcement, took Cadence's place and had her "subjects/minions" send a threat to Canterlot. Forcing Shining armor to use that Spell that weakened him so she could brainwash him with her magic. But this is all basic assumption on my part given the situation. It's all pretty Elementary, if i can compare myself shamelessly to Sherlock Holmes.

But this is all just my thoughts! I can't wait for Season 3 and I hope to hear more from you guys, and ladies. Also congrats on the new soon to be baby!