Proclus, whose quote opens the section on Euclid, lived more than 700 years after this date, well into the 5th century AD. Euclid himself wasn't born till about decade after, in 325 BC.
Unless gratefulness is actually binary (x is grateful for y), and directing this gratefulness towards someone is completely optional. (One might argue that the object of gratefulness is optional as well, and you can be grateful simpliciter, in an unqualified way. But to them I'd say there's an implied, general, object: the world, life, existence, or something like this.)
Correct. The translated sentence ("my son was a gude and honourable mon, but Sparta has mony a mon better than him.") does ends with two distinctly Doric words: τήνω (there, i.e. in Sparta) κάρρονας (stronger/better), instead of the Attic ἐκεῖ κρείττονας.
There's a very interesting comparison to be made between Wittgenstein and Spinoza, but saying something like "basically just a rehash" completely missed the point. Saying that Spinoza is "just a rehash" of Stoic ideas is a bit more correct, and still not very interesting outside of a very specified discussion.
The title "Tractatus Logico-Philosophicus" was not Wittgenstein's idea, but Moore's.