1) We have optimized for JSON. SOAP may work but I bet you'll hit trouble as we don't test/use SOAP responses since they are so much slower to parse compared to JSON
2) 1.1 and 1.2 but we use 1.2 in almost all cases for our calls. 1.1 should work and I've used some external clients but it's not the most test path.
3/4) No WSDL or XSD. The only docs are the docs in ZimbraServer/docs/soap*.txt |