For the California amendment, just the 540X will suffice (with an explanation in part II). For the penalties and interest, you could leave those blank and let the state bill you (checking that their amounts look reasonable before paying, of course). Otherwise, here’s their penalty info (yours will be the one for paying late, but not filing late), and interest guidelines.
Regarding your last question, you should provide a statement in addition to the 1040X itself, as Mary said. (I would actually print out an official Schedule B and fill it out with the foreign payer’s info, but since you’re adding a little income they otherwise wouldn’t even know about, the government probably won’t be too picky about your reporting!) Thanks @cathiexu!
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