You really spent a good bit of time on this, huh? Well, for your efforts, I feel I owe you an answer!
Assuming this scenario begins immediately after everything you described, I would probably do some resource hunting, and probably speak to some certain people. If I had to face the Princesses, I would most likely hunt down these things:
-The Staff of Sacanas, to drain their magic.
-Obsidian Orbs, to freeze them in place.
-Dark Stone to mold into armor, just in case.
I would then start an underground group of people who have either witnessed what happened, or believe it strongly enough. After a bit of planning, I would storm the castle with alongside the aforementioned group, steal the Princess' magic, and then enter the mirror. This could go a couple of different ways, so I'll branch it off from here:
1. The Princesses are Innocent. If this was the case, than I would undo the damage done, and most likely flee from Equestria, in hopes of finding my way back to Earth.
2.The Princesses are Guilty. If this were the case, than I would prove it to the people of Equestria, and have the Princesses banished to God-knows-where for their actions.
So, yeah... that's what I would try to do.
Well, that was an interesting half-hour...