Troubleshooting End User Issue

I am working on a series of 8 modules in Studio 13.  Five of them have been completed and tested both in Scorm Cloud and in the LMS (Relias).  Seriously, each module has been tested a minimum of 20 times each by a variety of people using a variety of hardware, software and internet connections.  Articulate support has also tested one of the modules and found no problems.

We did an early release of the first five modules and are getting reports that folks are getting stuck mostly (but not exclusively) at the quizzes.  Articulate support suggested that there is a known issue with players using HTML5 when there is audio feedback after the quiz questions.  But, we've experienced the sticking with Flash based players, too.  (And if that were the cause, wouldn't we have seen it during testing?)

Here are the variables I know about (I am an outside designer, am only receiving reports because I am not on-site, and have no expertise with LMS and systems issues):

• The problem is occurring (so far) only in this one program, but not everyone in the program is having a problem - about 20% are experiencing the problem
• All of the users in this program are up to date on their system specs (they can use whatever browser they want and the problems occur across browsers)
• There seems to be a lag in communication between the player and the LMS - for example, one user finished the course (listened to the very last bit of audio before closing the player) and the LMS showed that he was only 1/4 of the way done and when he clicked resume it brought him back to near the beginning
• When a user gets "stuck" and someone else in another location with good internet connectivity logs in as the user, that new person experiences that "sticking point", too. 

So I have 3 questions:

1. What percentage of the end users are likely to experience problems using the courses when we have done all that we know to do?  (Put another way what is an "acceptable" or "anticipated" rate for end users to experience problems?)  For example, do you anticipate that 80% of your end users will have no difficulties with a course?  (I need to know when I can stop looking for the needle in a haystack.)
2. I am getting to the limit of my expertise in this area...how do I go about finding someone who can help troubleshoot that has a good understanding of LMS, technology and internet systems, and perhaps even some design knowledge?
3. I am working with Articulate support on this, but we are getting to a point of running out of ideas.  What else do you suggest I do to trouble-shoot this issue?