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I do believe you are correct. Strictly mechanical joint.
Agreed, but there's a problem re-assembling as original. The bottom end of the column will be sealed with another press-fit part, so accessing the nut through the column will not be possible without doing some additional damage. And, the thin aluminum at the top of the column is very likely stretched, which is why the two pieces separated. I'd bet that nut isn't loose at all. It's a silly design, not meant to be serviced.
Here's a pic of the short top column section from my 055ProB to illustrate the lack of access to the nut:
So, see what Manfrotto has to say, but if they're not agreeable to a free replacement/repair, I'd go with the JB Weld fix. Properly applied, that adhesive is damn tough to break loose.