You are in a spot where you want to know what is going wrong and where. To do this, first get to know the published service. Get their definition either in a swagger.json, a wsdl, or even a textual complete description. Second, set up a soapui project where you create the message to send to their endpoint and add a testsuite containing each request and response combination you need. Third make the soapui project serve you mockservices. Forth, create your consuming calls. Fifth, set up unittests for your calling services talking to the mockserver. This way you will always have prove, step by step, which part works and which doesn't. But this is kind of the ideal scenario, that only few projects succeed in maintaining.

For now, for Issue on Hand 1), do check the Console → Advanced → Set log levels, and set the level for JSON to Trace. Or follow Jason's advice to split the call and the export mapping. You will either conclude your export mapping is at fault and can correct it, or you have a perfect requestmessage and can tell the receiving system that they should accept this request and respond accordingly.

Issue 2 is very likely just collateral damage and will disappear once issue 1 is gone.

Hope it helps, good luck.