Think of SWIFT as a large Websphere MQ installation, distributed across several data-centers.
You get an IP access by installing a router, leasing telecoms connectivity.
You get access to publish/subscribe with an SSL certificate exchange.
You get access to send/receive to a specific node(Bank) with another SSL exchange.
It is important to realize nodes can only send messages to those addresses which have been trusted, i.e. keys exchanged.
So a bank (Bank-A) trusts you to send SWIFT messages, all you can do with these messages is make accounting debits & credits within that bank. e.g. please debit $100 from my account number 12345 to account 67890.
You can also chain the messages, something like, debit my account $100, credit the account of Bank-B, and send my message on to Bank-B.
The message to Bank-B might say (debit Bank-B and) credit John-Doe-account.
(It's the job of the treasury department within the bank to monitor & keep these nostro [other bank] accounts funded)
Perhaps not obvious, but banks don't have hundreds of correspondent accounts. So to send money from a small town bank in Peru, to another small town bank in Palau, there are going to be a lot of these back-to-back transactions. The pathways are determined by hardcoded rules - not by SWIFT - but by each bank in the chain.
So sanctioning SWIFT is one part. The other part is getting every bank on SWIFT to remove all correspondent banking rules going via Russia. (Quite nice work I suppose).
Since the messages themselves are just XML, and not partially large, you could easily put all the days messages which got rejected into a .zip & use email (I suspect this is common operationally anyway).
Any kind of sanction on SWIFT would instantly lead to private MQ (or whatever flavor of messaging) infrastructure springing up everywhere & total chaos for every bank in the world (i.e. quite different to Iran).
This would cause a reduction in the number of correspondent accounts each bank would be willing to maintain. So longer pathways, slower international payments.
I doubt SWIFT could recover from a sanction. It would just close down. The endgame being heterogeneous networks, poorer monitoring, so the USA lose.
The sanctions put on individual Russian banks can be monitored (by SWIFT). In likelihood they are not used as correspondent banks.
>How does the system guarantee that nobody's creating money without notifying everyone?
SWIFT doesn't do this. I suppose a special audit is triggered (see Wirecard ) and the Auditor should catch it.