I can potentially help with one of the issues, I was also getting "Proxy username is empty" in the ta_nessus.log.
I thought this was strange because I remember putting the Access and Secret Keys in, but when I went to check the settings in edit, I could see that they were missing, so I put them back in.
I think if you go into the Inputs section and go to Edit as an action, you will have to fill in the Access/Secret key each time, it doesn't remember them.
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