Laurel put two and two together one or two episodes ago and realized Sara must be the Canary because of all her scars, her closeness to Oliver and their lame excuses for all their injuries.
Thea and Merlyn- there was the sound of a body falling, but when we get the reveal I could see Merlyn maybe on his knees, opening his suit to reveal his Kevlar with two impacts in it, and saying, "Ok, I can see that you're serious." Then a smile, and "You're more my daughter than you realize."
I thought Ravager was scary too, but they are really playing around with things now as far as translating the comics. Ravager in the comics has been Slade's daughter, whereas if I remember correctly Isobel Rochev was a much older woman who called herself "The Queen" after she took over Oliver's company. Given that much of a change it seems to make it less sure that Laurel has to become Black Canary. Not sure if Summer Glau has played a villain before but she has played a Terminator which can be programmed to be anything.