What does it mean to send "one sentence" to the future? Does the future understand English? How precise of a concept can we give them?

Typically, I interpret that as "what concept would you like people to have instinctually?", which most people respond to with "here's something that I wished my contemporaries understood, and so I'm going to inflict it on imaginary future post-apocalyptic humans," apparently because they haven't suffered enough.

When was the atomic hypothesis confirmed? If I recall correctly, it was only when chemists started noticing that the outputs of chemical reactions tended to factorize a certain way, which is to say that it took millennia after Democritus to get the point where the atomic hypothesis started making clearly relevant experimental predictions.

Mmm. I don't think it's that terrible, but it does appear designed to put people at Alhacen's level on the right track, rather than to put people at Aristotle's level on the right track. For example, I seem to recall the correct understanding of heat being held up by the atomic hypothesis not being widely accepted; if you know that atoms exist, then calculating everything correctly is fairly straightforward- but you do need some math to get it right. Feynman misses the opportunity to hint at electricity except very obliquely.

"Stop trying to sound wise and come up with theories that make precise predictions about things you can measure in numbers."

Why start off with the negative? Galileo's quote seems to capture the same idea more positively:

Measure what is measurable, and make measurable what is not so.

As for macroeconomics, money is useful, but only insofar as it facilitates trade. My first try would be:

Human skill accumulates because of specialization, returns to specialization are capped by the size of the market; trade increases wealth by allowing greater specialization which allows greater skill.