PROBLEM SOLVED!!!!!!!!!

Ever have one of those real forehead-slapping moments? I just had a big one.

Turns out the problem wasn't with GameStudio (JCL, you'll be pleased, maybe, to confirm that there appears to be no entity-limit problem in WED like I described as seemed to be happening to my project, nor any difference between version 6.3 and 6.4 (or 6.5) as was the coincidence when my level-loading issues started), and it wasn't anything wrong with my level, per se.

The problem was my SCRIPT. As soon as I removed it from the Map Properties, I was able to load my built wmb with no problem. This has been a problem for me for the better part of a year. Yes, I'm that dim when it comes to scripting, I freely admit it. I'd cobbled my level script together from recommendations from others and from the wiki, and parts seemed to work, and then I must have added something that killed the level load and I didn't realize it.

So, anyway, I can now resume work on my project, full-steam ahead. 2 years' worth of work NOT down the drain is a huge, huge relief...