Regarding a tie vote for Sunday night, which I believe is very possible, the last time they had a tie vote they did not go by previous votes cast against a person. As I posted earlier last night, the last time there was a tie at tribal council, it was decided by rock picking. If I remember correctly, it was down to the final four. The tie was between Neleah and Kathy. There was a chance again for the four people to come to a different vote and vote one out. Vescepia had immunity. I think she had a pact with Kathy at the time and would not vote for her. Pascal would not switch his vote from Kathy to Neleah. So, the old bag of rocks was pulled out and all tribe members had to pull, except for Vescepia with her immunity. Pascal was the one who pulled the purple rock so even though he did not have a vote against him, he was booted out right then and there.
With the Rupert/Jenna and Rob/Amber alliances, I see a tie on Sunday night no matter who wins immunity and everyone will be at risk (excepting the necklace wearer). Unless, as someone else pointed out and I missed it on the show, Jenna feels her alliance is with Rob and Amber and excludes Rupert.
Immunity will be a BIG factor, as always, when it comes down to these final four and then final three.
I am still rooting for Rob.