As far as not playing in IE 9, it seems to be a return of the same problem on this thread where it works only in compatibility mode in IE 9, the problem is not limited to mojoPortal and while I was able to get it working before it seems maybe it broke again from browser updates or something. I do not have an immediate solution for this but will look into it, I keep hoping for an update to jPlayer to solve these problems.
Some problems may also be caused by server configuration of mime types as noted in the jPlayer documentation. Encoding of the files can be a factor as well.
CSS of the skins can also be a factor.
In my testing of our demo site it works fine in Firefox, in Chrome when using the Artisteer skins in Chrome once I go to full screen I can't get back out of full screen. I do get the error you mention in IE 9 but it works if I change to compatibility mode just as in the past on the other thread.
Support for posters was not implemented in our video player but is something we may do for a future improvement.
Wish I had a concrete sure fire solution to offer but I don't.