COMPLETE SUCCESS!! Tony, thanks so much! As I'd wanted, the array with dual dropdown lists now only accepts sub-questions when responses are chosen in both dropdown lists, and survey-takers can navigate back from subsequent questions and change their dropdown list responses back to PLEASE CHOOSE. As a very big array and lots of subsequent conditional-dependent questions is the crux of our survey, this is just what we needed.
Since we didn't even need to consult someone knowledgeable in JavaScript to incorporate the scripts you provided (I hadn't realized that they could simply be plugged into the question editing box), I'll describe the process we went through, for the benefit of forum members whose level of programming naivety matches my own:
First we googled "limesurvey javascript" to learn how to incorporate the scripts, and found
www.limesurvey.org/manual/Workarounds:_M...28eg._JavaScript_etc
Scrolling down to "How to use Script (eg.JavaScript etc.) in LimeSurvey?", we followed the instructions to go to Global setting --> Security and set "Filter HTML for XSS" to "Off"--though the actual choices were, in fact, "yes" or "no", not "off" or "on". So we set it to "no". Then we went to the editing box for the array question, clicked on the "Source" button (the icon looks like a sheet of paper with lines of text), copied and pasted both of Tony's suggested scripts into the box, saved, and tested the survey. That's it.
Now I have a few questions:
1. Out of curiosity, after successfully testing the survey with the inserted scripts, we re-set the Filter HTML for XSS to "on", thinking the scripts might then no longer work. We were wrong--it seemed to make no difference either way. Can you tell me anything about the function of this filter? And what's with the on/off versus yes/no?
2. What's the best way to help out other LimeSurvey users, other than my simple description in this forum post? Should I post these as work-arounds?
3. You agreed that the issue of only one set of dropdown lists allowing a reversion to PLEASE SELECT is indeed a bug. Is it enough that you now know about it, or would it be helpful if I filed a bug report?
4. What about the issue of acceptance of partial responses? Is that considered a bug, or something to for Feature Requests?
Again, many thanks!