3yo old post here, but I just DM'd the original poster, as I had the exact same problem with my 7ft MF 18 scraper. And for 3-4 years I couldn't find any answers either. The correct answer is amazingly and stupidly simple. So for anyone else that might have been similarly and possibly embarrassingly mystified, here is how it works and what I sent to ALLDodge.
Greetings, I had the exact same issue, and found your post online probably a year ago, but you didn't get any correct answers. I saw it again today, and thought I'd reach out, in case you were still struggling. We found a workaround way to fully lower and lock the scarifier bar. It worked but it was far more work and difficult than it should have been. We'd lower the scraper onto 2 cinder blocks, and then wrestle the bar into position, but I knew we were probably doing something wrong. It shouldn't be that difficult. The intended method is of course ridiculously simple and "effortless" according to the manual I just found. With scraper on the ground, lower (rotate) the scarifier bar with the trip lever, then reverse your tractor a few inches, the scarifiers will latch into lowered position. That simple LOL!
The manual also solved another mystery on the center pipe and spring pin over the blade. Hope this helps hit me with a DM if you want copies of manual.