Hey Twilight! I hope you don't get discouraged from this game and play again. Being social enough is a problem a lot of new players struggle with, but it's a very fixable problem. Not too many people do well in their first MS game.
I usually try to have a "hook" in each PM I send, like an open-ended question. You want to make it easy for other people to reply to your PMs. Ask yourself how easy it would be to reply to if somepony sent it to you.
Some good topics to talk about:
-The theme
-Any twists that are in the game
-Your interests
-What's going on in your lives
-Literally anything else. Daddy and Derpy were even talking about the definition of art, lol.
Note: I haven't seen any of the PMs you sent, so I don't know how applicable this is going to be.
I empathize with you not knowing when to start talking game. If you start talking game too soon, you're seen as shady. Too late, and everypony's already bonded with each other. A good rule of thumb though is that by the time the immunity challenge is posted (and probably even earlier), you should know who's getting voted out if you lose. And if you don't know who it's going to be, then it's probably going to be you.