FWIW, I've had good luck with OSS/HuxWrx customer service. They have been above and beyond level of responsive.
As to your particular conundrum, I would experiment a little, if I were in your shoes, Ron. One of the first things I would try is a standard crush washer backwards (with the wide end against the shoulder), and see how things line up with an alignment rod. I used a $6 one from McMaster-Carr that I beveled the edges on with a Dremel. I bought the 0.212" one based on a recommendation I read somewhere (don't recall exactly where), giving a 0.006" clearance based on a nominal bore diameter of 0.218" in standard 5.56mm barrels.
Available here:
https://www.mcmaster.com/catalog/129/4218/8893K191
If that doesn't work, I would then try the thick shim which came with my OSS muzzle device, (the shim I warned about, not sure if it was in this thread). The shim isn't perfectly flat and can cause a misalignment on a square shoulder, but may be just the ticket if used with the slightly concave side against the Mini-14's shoulder.
Last, I would trim off a section off the bottom of the Mini-14's muzzle device, and either have it milled flat on the cut end, or work it on sandpaper over a flat surface in figure 8s until flat. I would then use that as my locknut.
One of these methods will work, if you're willing to put in the time and focus. Alignment must be verified with the alignment rod without fail though.
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