Look like we are trying to sum up our finding as to the OP from information that we already know. There will always be differences in how we see this, I have no problem with that…
I thought that I’d try the same endeavour, take of the turban off and try and start again to see where you’re coming from. It may help since two heads are better than one… or so they say

The problem with trying to be ‘objective’ about Islam is really that all the evidence is written by Muslims, there aren’t really any accounts from the pagan Arabs etc. You simply can’t escape the Muslim interpretation. It might be that we look afresh at the evidence to avoid apologia etc, but even then we’d have to ask if the Muslims were really that embarrassed with something they’d simply not record it.
If we start with the Bukhari hadith then with any book we have to ask who the author was and why did he write the book. Bukhari knew 600,000 ahadith and he said that 100,000 of them were rigorously authentic (sahih), but he only included 7,000 in his book. For a start we realise that it is only a portion of the ‘story.’ The book was written as an aide memoir for experts, which is why little explanation is included, which makes us realise we are getting an even smaller portion of the ‘story’ and that we may easily fall into error since he has assumed we have studied hadith, tafsir, logic etc.
The hadith about Ka’b comes up in (I’ve used A. Bewley’s excellent trans. found at:
sunnipath.com/library/Hadith/H0002P0000.aspx) book 61 ‘jihad and experditions’ numbers 2867 and 2868. Since the vast majority of experts reading this book are jurists he’s titled 2867 with ‘lying in war’ and 2868 with ‘assassinating the people of war.’ Therefore we can deduce that this event took place in war time and that in war time lying and assassination may be legislated. It also appears in full in book 67 ‘Expeditions’ number 3811. Here Bukhari has tried to put events in chronological order, this hadith comes in the section titled ‘This happened at the beginning of the six month after the Battle of Badr and before Uhud’ which means we can deduce its time.
The motivation given in the hadith is ‘he has insulted Allah and His Messenger.’ This statement could mean that Ka’b said derogatory things about Islam and Muhammad (pbuh) and so it was a ‘revenge’ type attack. This interpretation would seem unlikely since Bukhari also has the following hadiths:
“[The people of Tai’f] struck and wounded him, and he said as he wiped the blood from his face, ‘O Allah, forgive my people. They do not know.’”
“The strong man is not the one who throws people in wrestling. The strong man is the one who has control of himself when he is angry.”
“A bedouin urinated in the mosque. The people moved towards him and the Prophet, may Allah bless him and grant him peace, said, ‘Let him be and pour a bucket or pail of water onto his urine. You were sent to make things easy and not to make them difficult.’”
“And you do not do evil to those Who do evil to you, but you deal With them with forgiveness and kindness.”
The Qur’an also states: “those who control their rage and pardon other people. Allah loves the do good-doers,” (3:134) and “Good action and bad action are not the same. Repel the bad with what is better” (41:33). Just from these evidences we can deduce a president established of not doing things out of anger and it seems Muhammad (pbuh) was someone in control of his anger and did not retaliate. There are plenty of other examples, like the one of Abu Lahab where the response to an insult was a witty retort (we consider it revelation chapt 111). In fact there are many examples when Muhammad (pbuh) showed restraint, (sticking just with Bukhari) such as when 'Uqbah ibn Abi Mu’ayt threw a placenta of a camel on him and he simply carried on praying. So the question is, why is the Ka’b situation so different that it required assassination?