Yep - that stuff about the standard gauge to Marree all sounds like what I have read online as well from various historic sources. As you say Marree cattle trade couldn't keep the line open so it was truncated to terminate at Leigh Creek coal mine and this was a dedicated coal line from then on. I do go to the Leigh Creek area sometimes so can confirm that the standard gauge was pretty well built but apparently never got concrete sleepers upgrade, the few major highway level crossings have had all signage and crossing infrastructure decommissioned now, I guess the other issue would be any post-2016 erosion or settling of the ballast and elevated sections and the conditions of culverts and bridges etc, all the bridges are fenced off for safety reasons, but most of the infrastructure would still be there i.e. rails themselves. I'm guessing the balloon loop at the Leigh Creek coal mine end is all still there but not sure about the Pt Augusta end but we know it all joins the standard gauge network at Pt Augusta anyway. The line is pretty much how it was left in 2016 but with more weeds now obviously, I don't think the rail line is maintained to any standard as the access roads alongside this rail line have been washed out in numerous places and not graded for several years, so inspection of the line even by 4WD would require a bit of grader work first.
Some of the historical stuff about Broken Hill is hard to find, your info about Lithgow coal being used for syngas production sounds plausible to me.
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