For testing purposes I have and SSL certificate set up on a dev site where the domain does not match SSL domains valid for that certificate.
Would it still be possible to validate credit cards with Authorize.net even though the certificate domain is invalid?
It depends on how you are communicating with authorize.net, but when I did it, I sent a request from my server to their's to authorize the transaction. So, they never saw my SSL certificate, because they never sent a request to my web server. So, to answer your question, assuming you are authorizing the transaction by initiating an SSL connection to their server, then it shouldn't matter one bit if you have a valid SSL cert or not.
That of course was the technical answer. As for what their rules might be, I couldn't say.
Yes. This is because the the data being transmitted to Authorize.Net is being sent from server to server which is not using your installed SSL certificate. So although the user might be seeing an invalid certificate Authorize.Net will not. Plus they do not care if the certificate is with the proper domain name as long as they data is encrypted on your end.
Of course a better even answer would be to try it out. That's the best way to know for sure. Sign up for a developer test account and run some test transactions.
Authorize.net's code library has several parameters you can set to indicate you are doing a test transaction, in which case it shouldn't worry about your ssl cert. Would probably depend on which of Authorize.net's payment api's you're using though. I know this is generally how it works with their CIM library.