It is stated that the destruction of the Other Side will cause every denizen trapped there to pass on, however, the full reason why Silas' return was heralded as hell on earth was that the destruction of the Other Side would cause the resurrection of its denizens
In Season Four, Silas didn't want to destroy the Other Side—he wanted Bonnie to use the Expression Triangle to permanently lower the veil between the two dimensions so that all of the dead supernaturals could essentially be resurrected. He didn't actually seek out the destruction of the Other Side until Season Five.
Markos states in the story that the universe is responsible for the doppelgängers being drawn together is actually a misinterpretation, and that he is responsible for it. However, the original version of the tale was given by Qetsiyah, who was heavily involved with the creation of the doppelgängers, claims to have watched them come together over 2,000 years (500 of which were before Markos was born), and was intimately familiar with their tale, making it extremely unlikely that she would have gotten it wrong.
Damon states his exact age for the first time as being 173 years old, and this episode takes place in the year 2012, according to the show's timeline. However, in the episode,Bad Moon Rising, Damon states that his age is "160-some-odd-years," but never states his exact age, and this episode takes place in 2010. Presuming at the latest that Damon was 169 at the time, even two years later, he should only be 171 or younger, not 173.
Silas's compulsion on the townspeople and the gas station clerk should have ended either after he became a witch or after he died, but it's unknown if it ever was. It's possible Silas' compulsion never broke due to his unique immortal powers, but even so, his compulsion should've also been released when the townspeople crossed into the Travelers' anti-magic zone.
It has been established that Silas' mind control is different from vampire compulsion since he can use his mind control on any being and vervain does nothing to prevent it. However, it is correct that his mind control should still be considered magic regardless.
Stefan finds Katherine's suicide letter which talks about running from her enemies. When Katherine is about to jump off the clocktower, her voice reads the letter, however, she only reads half of the letter out, the letter actually reads:
Dear Nadia, Sorry I had to kill your boyfriend, but it was the motherly thing to do. Suicide, however, not very motherly of me. I ran from my enemies for 500 years, and then one day, I stopped. I stopped and I realized I didn't have a purpose anymore. [My entire] existence [revolved] around my survival and I'd...all my enemies. Now a new enemy wants me dead. Sure I could run from the Travelers, but there's still one enemy that I can never escape, time. So call it pride or vanity, but after the life that I've led, I refuse to drift off into the sunset. Good-bye, Nadia.
Note:Text in bold is parts Katherine's voiceover missed out. Text in brackets is what we think is beneath Stefan's thumb.