I discovered that my problem was due to using a third party component that turned out to have slightly different methods for its class, so it worked the same for 99% of things, but not for that one issue.... Now I'm using Flash Pro 8, which has solved many of my problems.
That being said, in Flash Pro 8, the Scrollpane component has a scrollDrag parameter, which causes the same behavior I described when enabled. You can set it to "false" if you edit the parameters manually for the instance on stage, or if you're adding the scrollpane progmatically, it should be something like:
scrollpane.scrollDrag = false;
Did you try that? I'm thinking that could be causing not being able to use the input text, but if the populating function isn't working, I'd think it's something else.
Since it works exported to Flash 7 when not in a scrollpane, is it just a relative paths issue? In the project I'm currently working on I forever refer to everything with the _root.scrollpane.content. targeting to make sure I'm addressing the correct objects....
Another interesting issue I ran into is that a lot of solutions suggest using refreshPane() to make sure the pane is updated correctly. But when it points to an external SWF and you dynamically create text fields or movie clips, it seems that refreshPane() ends up wiping them out and reloading the SWF specified in the content path. So if you programatically set your combo boxes etc. and then refreshPane(), try commenting out the refreshPane() and see what happens.
You might also want to trace the value of the text boxes or combo boxes after the actionscript first sets them. I ran into a quick-fix issue where I was setting the contents of a dynamic text field and it was losing the default text format and instead showing nothing.... Tracing it showed the value was stored, so I realized I just had to assign a text format with setNewTextFormat().
Not sure if any of those are the cause of your problems.... If you still have issues, perhaps you can post your FLA?