Perry, I probably don't have a good suggestion to actually help you, but I will give my own perspective.
On my Ning site, we have quite a few very long running threads in our Forums that members enjoy. The Positive Thread (to talk about uplifting things in one's day), The Drifting Thread (to discuss all non-music related stuff as you please), the Show Us Your Pets Thread (to show and tell about your pets).
These threads go on for months and accumulate HUNDREDS of posts...many with attachments or media embeds. And many pages.
I have my whole forum on ning set to NON-threaded, and this works just great for my site. In fact I've chosen non-threaded for my JR forums as well. When members are posting in reference to something previously posted, they simply use the REPLY button which includes the previous material as quoted, which they can trim as needed.
Here is one such long thread, to show how it comes out:
http://mountaindulcimer.ning.com/forum/topics/the-drifting-thread-1?x=1&id=3745489%3ATopic%3A630530&page=12
(note this one is 'only' 17 pages long still, since I wiped it clean a couple months ago during site Spring cleaning.)
I realize your site and my site are different. But I'm wondering if you have considered whether being so attached to threaded discussion style is actually worth all the difficulties you are having to go through to keep it that way? Is having threaded forum posts a thing worth going down with the Ning Titanic over?
While building my JR site, I've come across a few crossroads where I decided to adapt to a slightly different way of doing things....things that I used to think were absolutely essential, but upon thinking further I realized that some were things I was just being too attached to and inflexible about.
I hope you'll get some other responses with more helpful info than I have for you.
I only mention this because I've had to make some decisions for my own site that were upsetting for me at first, but now months later and getting close to launch those things seem less crucial compared to having my new site. My site won't be exactly like my ning site...but now I'm kinda glad about that, and am ready to have a rebirth!
Meanwhile, I watch the undeniable signs of both ning platforms crumbling away, day by day.
--
...just another satisfied Jamroom customer.
Migrated from Ning to Jamroom June 2015