You shouldn't let iMovie HD 6 be the reason for you not to upgrade your OS. Simply put iMovie HD and OS X 10.8.5 on a separate partition on your hard drive or on a separate external hard drive.
We have a bunch of Mac Minis dedicated to the sole purpose of editing movies here. Naturally, we are using iMovie HD 6 (and iDVD) because we believe it to be the best. We are running OS X 10.9.5 on these Macs. I believe that OS X 10.9.5 is the last OS version to run iMovie HD 6 with no issues or glitches.
I do appreciate the efforts made by others to run iMovie HD 6 on later versions of the Mac OS. It's really a crying shame that Apple abandoned this wonderful program.
If it were up to me, iMovie HD 6 would have been updated and improved with full support for burning Blu-rays. For several years now, Apple has been hostile to optical media. However, optical media is now making a massive comeback with the advent of M-DISC, because of its 1000 year lifetime.
Governments and industry alike are now utilizing optical media, (M-DISC) because it is considered the most stable and longest lasting data storage that exists today.