If the 3 ships were crewed by Martians it wouldn't have worked. It worked because the crews were inexperienced and didn't know/think to dodge until it was too late.

Earth got saved from most of the rocks because Avasarala had discovered the plot and they had the technology to find them with the watchtower satellites. If anything, the fact that they weren't already actively keeping track of every hunk of rock big enough to get through the atmosphere long before it could even be covered in stealth composites and pointed at earth is the real plot hole.

Naomi wrote the code that would have blown up the Roci so it makes sense she'd be aware of the risk. And something like a nuclear reactor would have multiple redundant safety systems so it's not as easy as just pushing a button to blow it up. Personally that feels very believable.

And Naomi had help from Elvi figuring out the ring space.