It is likely that some of the objections from 'northern brethren' came from convention slave owners Carroll and Chase from Maryland, Franklin from Pennsylvania, Hancock from Massachusetts, and Jay from New York.
Don't be concerned about the clause deletion....it is a bunny path. Its importance lies with the fact that Jefferson paid more than pay lip service to cessation of the slave trade.
“I don't find anyone saying that Adams killed or tried to kill the passage either in the committee or in the Congress.”
The next year Adams, speaking out against a bill to emancipate slaves in Massachusetts, said that the issue was presently too divisive, and so the legislation should “sleep for a time.” (Henry Wiencek 2004).
As others have noted, the Declaration wouldn't have changed anything about the slave trade one way or another. Jefferson didn't fight for his clause either. In this as in other things he was more about poses and appearances than following through.
Of course, Congress voted to end the importation of slaves from abroad while Jefferson was president. The Constitution allowed Congress to do so after January 1, 1808, and it would probably have happened whoever was president. Jefferson deserves some credit, I guess, but one also has to take in to account his later views on slavery if one wants to pass judgment on him.
The next year Adams, speaking out against a bill to emancipate slaves in Massachusetts, said that the issue was presently too divisive, and so the legislation should “sleep for a time.” (Henry Wiencek 2004).
That was actual abolition which went beyond what Jefferson advocated at that point. It was also wartime, and the fate of independence hung by a thread. After the war was over Massachusetts courts ruled slavery invalid based on language in the 1780 state constitution that was largely drafted by ... John Adams.
If Adams was wrong in 1777 he was wrong, but through most of Jefferson's own career Thomas Jefferson also very much wanted anti-slavery legislation to "sleep," even favoring the expansion of slave-owning territory under the specious theory that diffusion would make the institution weaker.
He was a complicated man with different facets in his thought and character, some admirable, others not. I guess you can say that about many people, but it's something "Jeffersonians" haven't always wanted to face up to.