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For #1, there are three points of evidence in favor of the 10-foot reach (and none that I can find in favor of a 5-foot reach). You can find the reach of Large-sized (tall) creatures under Table 8-1 in the CRB. We know Dragonkin are tall from their entry in AA. Elsewhere in that section it describes that "Creatures that take up more than 1 square of space typically have a natural reach of 10 feet", which would mean that 10-foot reach is the norm when presented with an unknown. Lastly, a Xenomech's "size category changes to match the chosen race". Where are the details of that size found? The same sentence that tells you Dragonkin have 10-foot reach.
For #2, I agree that seems like a typo.
For #3, nothing in the Xenomech's description allows us to choose alternate racial features, and those alternate racial features were introduced in that same book, so it's easier to assume that's intended. With Starfinder being a permissive rules system, you would need something clearly indicating that alternate racial features were available for Xenomechs (such as a ruling from your homegame GM, or in the case of Society, a clarification from Campaign Leadership).
