Let's step through this...
B. Dontos was being paid by Littlefinger to kill Joffery.
C. Olenna placed the poison and was in on it.
D. Varys likely knew of both the poison and Sansa's escape.
I haven't seen solid evidence of any of these three. Varys might have known of the poison and Sansa's escape, but I doubt he knew both, let alone either. As shown in this episode Little Finger had the necklace made. If it did contain the poison and Olenna was involved we're now talking about a conspiracy of 3, if not 4 people including a known drunk. As Little Finger points out, you can't trust a drunk to keep his mouth shut.
Most likely Dontos was paid to give the necklace to Sansa and get her to the ship during/after the wedding. It doesn't take a genius to figure out that death of the King during the wedding is a great time to get her to the ship.