Emeyr, in each of your examples, a native american English speaker would most likely choose "could" rather than "can".
DL's sentence is grammatically correct, "Can" can be used to express a "generalization" as stated in the last section in the above link. (Native of Boston, Massachusetts and ESL teacher.)
Yes, of course, it is grammatically correct. It just isn't the most likely way native speakers would choose to speak. Of course, (and I want to be clear, this is intended to be a humorous jibe) amongst we midwesterners, there is a lingering suspicion whether Boston and other Eastern cities still speak American.
Does this really mean "they can suffer an accident"? or is there an alternative translation.
As it shows a possibility, poder would be better translated as "may/might"
Yes, "They could have . . ." "They might have . . ." and "They may have . . ." would all be better translations than "They can have . . ."
I think that they're complaining about ‘can’ here, not about ‘suffer an accident’.
They can have an accident if you like, I don't care. Just as long as they're dead when you're done.