after monkeying around with this for a while more, it appears that the gjallarhorn plan can be migrated to more of a straightforward fold while preserving a substantially similar performance in many ways, improving in others, and simplifying the fold plan.
subject to further revision and refinement, this is where it stands currently, and all feedback is appreciated. i'll need to double check everything again, but I've already double checked several times of course.
the size of the unit is 30" high, 72" wide, 25.5" deep (or so depending on panel thickness used), but roughly 2.5' x 6' x 2'. obviously it is end firing, but as another option the mouth of the horn could be split into two pieces and that area removed from the side panel for something of a "dual side firing" option, or the mouth could just be taken out of the panel behind the driver.
'smushed' response of low profile horn (dark gray) vs. gjallerhorn (light gray):
architectural plan, bracing required (not shown):
dual side firing configuration: