"Cobra" is any snake in Portuguese, but a specific kind of snake in English, so I guess it depends. Interestingly, in looking this up I learned that "cobra" is one of the words in English that we get directly from Portuguese! http://oxforddictionaries.com/definition/english/cobra?q=cobra
Not a mistake in itself. But not showing "arrive(s)" would definitely be a mistake.
"Chegar" can be used in some contexts as "suffice" or "to be enough".
- Já chega!!! = That's enough!!! (typical upset sentence)
- Chega de enrolar, vamos trabalhar = Enough stalling, let's work