Items 1 and 2: AWS sends such emails to the email address associated with the root account, so choose that address carefully, and ensure that it is monitored.
There does not appear to be another notification mechanism. These two items are essentially the same as far as I can tell -- if the payment method is declined, the account is technically overdue, since payment is due immediately.
There are other good reasons to monitor this email account, too -- AWS has been known to go above and beyond their portion of the shared security model on occasion... for example, they performed a proactive audit and notified affected customers that they should verify whether they really did intend to bypass the secure default settings on S3 buckets and make them publicly accessible. (As you may have noticed from recent news items, some combination of lack of controls, carelessness, inexperience, or laziness sometimes prompts people to configure buckets for open access.)
You can get also the monthly invoice emailed as a PDF and use this as a trigger to cross-verify that payment was successful.
Item 3 is a side effect of failing to react to the other two. An account that remains unpaid is subject to termination, presumably without further notice. The exact terms of when the services might be disrupted is not something for which I can find a citation.
Anecdotally, AWS appears to suspend access, somewhat piecemeal, nondestructively at first.
I've seen customers not pay due to an expired card, and the first side effect after some time elapses is S3 buckets returning an apparently undocumented error response:
<Message>All access to this object has been disabled</Message>
This appears to be the result of some kind of kill-bit that can be set for an entire bucket. The customer fixed the billing issue, service was restored, and no content was lost.
Additionally, I have one relatively large account whose bank repeatedly denied the recurring monthly AWS charge (~$5,000 iirc) as suspicious desipite the customer's request that the bank accept them as genuine. This never resulted in any service disruptions on the account.