"Newton related his ideas to Leibnitz in letters. Leibnitz developed the ideas, and published, but neglected to give Newton proper credit. That was the source of the conflict."
You're wrong. The claim is that Leibnitz may have seen some of Newton's unpublished manuscripts -- that while not spelling out calculus, would have given direction, hints.
Leibnitz admitted he had seen some of N's MS. And recently, IIRC, one of N's MS was found amongst L's possessions. But that still doesn't prove Leibnitz plagiarized. There was not enough in the MS for it to be plagiarism.
Leib published his calculus 20 years before Newton's came out (in the Optiks). It's hard to believe L got it all out of some MS that wasn't even on that subject, but at best suggestive of the subject.
No, it's far more likely that it was in the air--the zeitgeist. A lot of other people had been and still were working on the same stuff at the same time. These things happen.
Frankly, you might make a stronger argument about the laws of motion because Galileo made quite a lot of headway on those before Newton, and Galileo started from scratch. Newton reformulated them.
I'm not saying Leibnitz was a plagiarer. But he clearly had Newton's ideas, and they did exchange letters. Leibnitz would not likely have published what he did without Newton.
Personally, I think any one of Newton's contributions: gravity, laws of motion, or calculus were bigger deals than relativity, though I'm not suggesting that Einstein's contribution was small. It just wasn't as big as Newton's.
I would say that maybe Einstein's insight was more brilliant than Newton's were, but when you think of it, the laws of gravity really were a major insight.