For the "Array" issue, see the recommended patches list
For the "Failed AJAX request or invalid JSON returned" message, check your PHP error log to see if something appears there. If not, what browser are you using? You may need to use the browser's developer tools or a plugin to see if there is an error message getting served from PHP into your AJAX response.
For the occasional upload bug, is it possible that you could capture this in more detail -- is it reproducible? Also, the above suggestion to check your error log might shed some light here too.
For reviewers being able to see their review history -- the dashboard (including reviewer's archive view) is among the highest-priority things we'll be working on for the next major release.
For the workflow tweaks (reviewer self-registration to multiple presses and author assignment to reviewers) we haven't heard of that type of workflow before, but will keep our ears open for other similar cases.
Your final question about workflow users who handle reviewers: currently the Editor role is the closest match. We don't have something specific to just handle reviewers. However, you could try experimenting with editing roles in the Access & Security area of Management to see if that will suit you.
Public Knowledge Project Team