Enlightenment.... Here it comes.... To some extent....

Ok,

Leaving all the politics/trading aside.

1) Your first question was about the nature of the 'trust vote'. I'm pretty sure (around 95%) you're right that it's nothing more than a vote on the nuclear deal. This is evidenced by the fact that this 'trust vote' has been moved by the Prime Minister himself, and it is not being described as a 'vote of no confidence'.

Now, if that is the case, then there is a million precedents and there are none. There are a million precedents in the sense that it is only a normal bill being tabled which will be voted upon normally. But I can't think of any bill which was tabled and then voted upon which single handedly decided the fate of the incumbent government.

2) The answer to the first question will automatically answer the second. The fact that this 'trust vote' is a normal bill means that it will be decided by the a majority of the members present and voting (Article 100). So, as you rightly put it, they will need only 261 votes.

3) As for the party whips your informed sources (read- me) were right in that the whip applies to members of a political party.

But I'll explain how it works. A person is seen to have defected only if it is found that he has voted contrary to the party leadership's instructions without prior permission. When this happens a person is disqualified from membership in the house, so he cannot vote at all. I imagine that if such a situation arises there will be a re-vote in 15 days time, when the time period for condoning such a defected vote expires. So, there is nothing like 'openly defecting' before the actual voting takes place.